WO1994007737A1

WO1994007737A1 - Tag axle control

Info

Publication number: WO1994007737A1
Application number: PCT/US1993/009535
Authority: WO
Inventors: Grover H. Riefler; Monte P. Riefler; Lorin E. Hermann
Original assignee: Ashford Concrete Inc
Current assignee: Ashford Concrete Inc
Priority date: 1992-10-06
Filing date: 1993-10-06
Publication date: 1994-04-14
Anticipated expiration: 1995-04-06
Also published as: AU5352794A

Abstract

An improved operation is provided for a tag axle (8) for vehicles by providing a 'floating' mode of operation in which the tag axle (8) is in the ground contact position, but the load bearing suspension components are deactivated. A pneumatic extension bag (26) suspension and a lift bag (24) retraction system is preferred, as the venting of the system to provide the floating mode is far simpler than in the hydraulic case. The floating mode is actuated by the vehicle operator by shifting into reverse gear, and in response to tag axle overload conditions. Most preferred are camber actuated self-steering configurations. Substantial ground clearance is provided by a rear mount configuration in a tag-axle configuration with the tag axle assembly (15) mounted behind the rear limit of the vehicle frame to a shear plate (18) attached by a plurality of bolts (19).

Description

TAG AXLE CONTROL BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention is related to U.S. Patent 5,018,593, issued May 28, 1991, the disclosure of which is hereby incorporated by reference. The invention herein is directed to an improvement in the mode of operation, and in the configuration and structure defined and claimed therein. 2. Description of Related Art and Information

The present invention relates to the field of vehicles for carrying heavy loads, and particularly to improved tag axles for bearing and spreading a share of the load in high vehicle gross weight conditions in order to comply with highway and bridge load-limitation laws and regulations, and which retract to an inactive position when not required.

Tag axles in a variety of configurations are coming into increasing use on heavy load-bearing vehicles and related vehicles as a means to comply with highway, road and bridge load limits established by a diverse and complex array of laws and regulations. The dominant requirement for most purposes in the U.S. is the federal vehicle weight limits established by the federal Bridge Gross Weight Formula defined at 23 C.F.R. 658.5 and 658.17.

The federal Bridge Gross Weight Formula provides, in substance, that:

1. Maximum allowable vehicle gross weight is limited to 80,000 pounds (36,287 kg s) .

2. The maximum allowable load on any single axle is limited to 20,000 pounds (9,072 kgms) .

3. The maximum allowable load on any tandem axle assembly is limited to 34,000 pounds (15,422 kgms) .

-1- 4. The maximum allowable load on any sequential series of axles must not exceed a weight W, defined by the formula: , L x N _λ

W = 500 x + 12N + 36 )

N - 1

where W is the maximum allowable weight, in pounds;

N is the total number of axles in the sequence;

L is the length of the wheel base between the first and last axles in the sequence. The formula is made more complex by a series of definitions of single axles, tandem axles, and related provisions which govern the determination of L dimensions and other parameters.

The net effect of the federal Bridge Gross Weight Formula is to allow greater gross vehicle weights, up to the limit, both as overall wheel base is increased, and as the load is spread within that overall wheel base on several axles.

On the other hand, many jurisdictions have overall length restrictions on various types of vehicles. The common limit for a unit vehicle, i.e., a truck with a unitary non-articulating structure, is 40 feet (12.192 meters) overall length maximum in most states.

A complex maze of other requirements and limitations apply in various jurisdictions, and the result is a severe set of limitations on the payload of many types of vehicles. Compliance is increasingly difficult to assure.

Non-compliance brings high fines and penalties.

As one type of vehicle which is greatly limited by the regulations and laws, ready-mix concrete delivery vehicles are most often limited in payload by the federal Bridge Gross Weight Formula than any other parameter. In most circumstances, a typical three axle ready mix concrete vehicle will be limited to a legal capacity of

-2- about 6 cubic yards (5.4864 cubic meters) of concrete mix. Equipped with five or even up to six axles and lightweight mixer drum assembly construction, the legal capacity has been extended to from 8 to 8 1/4 cubic yards (7.1352 to 7.322 cubic meters). Such load spreading is not necessary or desirable in off-road operations, such as at a job site where a load of concrete is to be discharged, for example. As a general rule, the greater the number of axles, the higher the allowable gross vehicle weight, and thus the net pay load of the vehicle. On the other hand, adding additional axles to a vehicle leads to other disadvantages. Higher rolling resistance, losses in fuel economy, engine wear, steering and handling difficulties under some conditions, losses in traction by the driven wheels in some conditions, and all serve to make added axles difficult and expensive.

The use of a tag axles has the advantage of an added axle to eliminate most of these serious disadvantages. The tag axle is designed to be deployed in a down position to bear a portion of the load in highway service, and to be retracted to a level that is not in contact with the road or ground for off-road operations or for highway operation in unloaded or light load conditions when the weight bearing capacity is not needed. Lifting the tag axles or other forms of tag axles serves to decrease the load imposed on the front axle of the vehicle, making steering loads lighter, while increasing the weight and traction on the driving wheels, and shortening the wheel base to aid maneuverability. In addition, tire wear is reduced while rolling resistance and fuel consumption are decreased in off-road operations and in highway operating conditions in light load or unloaded conditions.

There are two major types of tag axles in use. These are under-frame systems, typified by the units available from Hi Steer Canada, Ltd., and so-called high-lift models, offered by a number of truck builders and

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SUBSTITUTE SHEET suppliers, of which our prior U.S. Patent 5,018,593 is an example.

The under-frame systems are typically pneumatic suspension and retraction mechanisms. Such "air ride" suspensions have been highly developed in recent years and have excellent performance characteristics. As tag axles, these systems are ordinarily based on pivotally mounted leading or trailing links, depending on the location of the assembly. When the link leads the axle, these units are known as "tag" axles. When the link trails the axle, often employed when the assembly is mounted ahead of a tandem axle pair, the unit is normally called a "pusher" axle.

As illustrated in our prior patent, supra, the high- lift systems are in tag axle configuration, and are normally hydraulically actuated. The suspension system may be hydraulic or pneumatic.

The under-frame systems limit ground clearance, and have limited lift clearance, ordinarily no more than about 18 inches (45.7 cm). It is often difficult to mount a rear under-frame tag axle in a manner such that the center is more than 96 inches (243.84 cm) behind the center of the next forward axle. As a major factor in the L measurements under the federal Bridge Gross Weight Formula, the benefits of a tag axle may be limited in such a case.

The high-lift systems are mounted above the frame, retract well above the frame and have excellent ground clearance, but are more prone to failures. Problems generally arise because of the length of the link arms, and because the suspension is provided by the hydraulic system in most units of this type. In prior patent 5,018,593, a separate pneumatic suspension system resolves a portion of the problem, but because the suspension is mounted on the link arm and not on the frame of the vehicle, the long lever arm of the links transmits the

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SUBSTITUTE SHEET entire load and are still a considerable problem. The weight of such units is undesirably but necessarily high.

For both under-frame and high-lift types of tag axle systems, there is one particularly graceless failure mode which is totally unacceptable because of the extensive damage to the vehicle which can result. It is to that particular problem that the present invention is directed.

When the vehicle passes across a depression of sufficient span and depth, it is sometimes possible for the entire weight of the vehicle to be placed on the foremost and the rearmost tag axle. With the vehicle in motion, the inertial loading of such a condition can be several times the static load represented by the gross vehicle weight, and failure of the tag axle or the vehicle frame can result.

Such conditions can occur with substantial frequency for construction vehicles and other combined highway and off-road vehicles. Typical of these is a ready-mix concrete delivery vehicle, for example, which operates from a ready-mix concrete plant to a job site over the highways, and while at the job site to deliver and discharge its load, is expected to traverse the site whatever the conditions may be. Frequently, there are occasions when crossing a drainage ditch, swages and other depressions in the topography on the site are required. If, through operator haste and error, the tag axle is not retracted to its lifted position, front-rear axle bridging may occur. If this occurs at a substantial speed, it can result in a dynamic inertial loading far in excess of the design requirements for such equipment. Gross vehicle weights of 60,000 to 70,000 pounds (27,215.5 to 31,751.4 kgms) are common to such vehicles. Inertial loading in a bridged condition can, in some circumstances, reach as high as 30,000 pounds (13,607.8 kgms) or more over a beam length, spanning from the front axle to the rear axle, of as much as 30 feet (76.2 cm). Under such conditions,

-5- failure of the tag axle assembly or the vehicle axle is inevitable.

SUMMARY OF THE INVENTION The present invention is designed and has the benefit of:

(1) preventing the occurrence of front-rear axle bridging conditions with the load borne on a rear tag axle;

(2) protecting the equipment from damage occasioned by front-rear axle bridging damage;

(3) protecting the vehicle from operator error in neglecting to retract the tag axle to its lifted position during off-road operations;

(4) relieving the load on a tag axle when an overload condition occurs by transferring the load to an adjacent tandem axle bogie constructed and placed to accommodate such overload conditions; and

(5) providing a lift tag axle having greater ground clearance than under-frame types but without the higher failure rates associated with high-lift types.

The present invention provides a suspension relief means to unload an extended tag axle without retraction to the lifted position which provides a "floating" mode of operation, where the axle is extended, with the tires in contact with the ground or road surface, but without bearing the load of the vehicle weight. Pneumatic suspensions are preferred for the facility with which they can be operated and controlled in such a floating mode, but the axle is also applicable to hydraulic suspension systems. With fluid actuated suspension systems, the floating mode is actuated by venting the suspension pressure, typically by a valve or other means for relieving fluid pressure, activated by the vehicle operator, or automatically actuated by a load sensing transducer at some predetermined load limit.

The best features of both under-frame and high-lift types of tag axles are attained, while the worst

-6- limitations of each are avoided. This is achieved by mounting a tag axle on the end of the vehicle frame rather than beneath it, raising the mounting assembly by as much as 12 to 18 inches (30.5 to 45.7 cm), and providing for comparable increases in ground clearance, limited only by the presence of other equipment in the rear overhang area of the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an improved tag axle connected to a rear frame member of a cement truck with the tag axle in the extended position on a flat ground surface.

Figure 2 is an improved tag axle connected to a rear frame member of a cement truck in the lifted or raised position on a flat ground surface. Figure 3 is an improved tag axle connected to a rear frame member of a cement truck in an extended position while crossing a ground swale.

Figure 4 is an improved tag axle connected to a rear frame member of a cement truck in an extended, "float" mode position on an irregular ground surface.

Figure 5 is a detailed view of an improved tag axle in the lift or raised position.

Figure 6 is a detailed view of an improved tag axle in the extended or lowered position.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Tag axles are of increasing importance to the trucking industry in maximizing pay-load while observing load limits such as the federal Bridge Gross Weight Formula. At the same time, tag axles have proven to be prone to excessive and unacceptable numbers of failures in service. Failures are commonly associated with operator error and neglect, such as operating in off-road conditions with the tag axle extended in its load bearing condition with damage to the equipment often quite extensive. Occasional failures are experienced in usual highway operations as well.

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SUBSTITUTE SHEET Increasing numbers of failures and problems with high-lift tag axles has led to reexamination of the design of the major alternative, the under-frame mounted tag axle assembly. The superior engineering properties of the under-frame mount are attributable to two major factors. The first factor is that most such units employ pneumatic suspension elements which, when problems arise, fail in a more graceful manner. The second factor is that the link members of the usual under-frame models are far shorter than those of high-lift models, and since they operate as lever arms, they transmit lower stresses on the actuating mechanisms under high load conditions because the lever arms are shorter. In some implementations, the suspension directly joins the axle and the vehicle frame, so that the link arm serves primarily to locate the axle and does not bear any significant proportion of the load. The major disadvantage of the under-frame types has been the limited ground clearance when the axle is in its lifted position. A distance of 12 to 18 inches (30.5 to 45.7 cm) is typical and, particularly for rear tag axle configurations, is inadequate.

In addition, when bridging conditions are encountered, such as crossing ditches, swages, and other depressions or abrupt grade changes, and the entire gross vehicle weight is borne on the front and rear axles, no existing type of tag axle is satisfactory, and failure of the tag axle assembly and/or the vehicle frame can result. This type of damage is not ordinarily encountered with the tag axle in its lifted or retracted position, as such loads are within the usual design parameters of the usual tandem axle bogie, which is mounted farther forward, providing a shorter wheel base, and which is engineered to accept higher load limits without failure. Tag axles cannot provide a gain in payload weight if they are built to the weight required to bear such loads. If that were possible, the vehicle frame would then become the primary failure point, which is entirely unacceptable.

-8- In the present invention, resolution of these problems has been achieved with three specific provisions:

(1) improvement of the ground clearance of the tires mounted on a rear rear-most tag axle from about 12 to 18 inches (30.5 to 45.7 cm) to about 22 to 36 inches (55.9 to 91.4 cm) ;

(2) provision of a graceful failure mode which would produce a limited and modest damage should it occur; and

(3) provision of a new mode of operation which can be either operator selected or automatically actuated under appropriate conditions to prevent most causes of tag axle failure from arising.

The usual construction of heavy vehicles is based on a ladder frame assembly, having two longitudinal box beams running the length of the vehicle, bridged and joined by a plurality of transverse box beams. This basic structure is strong, and is further strengthened by mounting other elements of the vehicle to the frame. In most cases, the frame is terminated by a transverse box beam. An under-frame tag axle is adapted to mount on the rear facing, vertically disposed plate of the rear transverse box beam rather than under the frame.

The tag axle assembly is provided with a forward facing, vertically disposed plate which mates with the box beam plate, and is fixed in place with a suitable number of bolts to bear the loads required. Such a configuration requires that the trailing link elements of the tag axle be lengthened, and that the suspension deflection be increased to accommodate the higher mounting position. By lengthening the link arm, the L measurement under the federal Bridge Gross Weight Formula is increased, providing a considerable benefit.

The entire tag axle assembly is raised as a result of this new arrangement by as much as 12 to 18 inches (30.5 to 45.7 cm) above the usual under frame mounting position. In addition, the moving parts can be retracted without interference by the frame elements, so that the range of

-9- the lift action can be extended beyond that normally attainable. As a result, ground clearances of 20 to 36 inches (50.8 to 91.4 cm), or even more, are realized.

In addition, the mating plates of the vehicle frame and the axle assembly provide another advantage. They define a shear plane which, by the selection of appropriate numbers and sizes of bolts, can be provided with a graceful failure at a load which is less than that at which the frame itself or the structures of the tag axle assembly will fail. It is preferred that the joint be designed to fail in shear at about 85% to 90% of the load which will cause failure of the tag axle assembly or of the vehicle frame, whichever is less. While some damage may be done if the shear plane joint fails, it will be far less than that which has resulted from typical tag axles, whether under-frame or high-lift types.

It is far preferable if the system can be protected from the conditions which produce failures, whether they arise from operator error and neglect or are encountered in service conditions.

The tag axles heretofore available have two modes and positions, each with fixed characteristics. In the lifted position, the suspension is inactive and the axle assembly, wheels and tires are clear of the ground, and held in an inoperative position and condition. In the extended position, the suspension is active, bearing a share of the gross vehicle weight, and transmitting it through the axle, wheels and tires to the road or ground surface. A third mode of operation is provided, which is designated the "floating mode." In the floating mode, the tag axle is in its extended position, with the tires in contact with the road or ground, but the suspension is inactive, so that no load is transmitted from the vehicle to the axle, wheels and tires.

The floating mode is actuated by venting the fluid which normally pressurizes the fluid suspension. A valve

-10- or comparable means for releasing the pressure to the atmosphere in the case of pneumatic suspensions, and to a reservoir in the case of a hydraulic suspension, is the preferred means for controlling actuation and de-actuation of the suspension system. Mechanical systems are more complex to deactivate, therefore, fluid suspensions are preferred.

The valve or other suspension actuator means may be electrically, pneumatically, hydraulically or mechanically operated. Such valves are familiar to those of ordinary skill in the art, and are widely available. Operation can be in response to a control signal, provided to a suspension actuator control means, such as a solenoid, electric motor, hydraulic or pneumatic piston, or the like. The control signal can originate from the vehicle operator or, to prevent inadvertent failures, from a load sensor or the like which detects an overload condition on the tag axle and transmits a signal. In either case, a signal causes the suspension actuator control means to open the relief valve or other suspension actuator means, placing the system in floating mode. The load is transferred to other axles, often a tandem axle pair.

The floating mode can be provided for a fluid suspension based tag axle, including the typical under- frame and high-lift models common in the art. The floating mode and its applications is not limited to the specific preferred tag axle mounted on the rear face of the frame, as discussed above. Indeed, the floating mode of operation can readily be retrofitted to existing tag axles of any fluid suspension type commonly employed in the art.

Relief of the fluid pressure takes a definite time before the load is fully transferred to other axles. The time required will depend on the volume of the load bearing pneumatic bag or of the active hydraulic cylinder, the fluid pressure, and the vent orifice area through which the pressure is relieved. It is accordingly

-11- preferred that the vent orifice afforded by the vent valve or comparable venting means be as large as practicable, as the other determinants of the rate of venting are ordinarily established by other requirements. It will generally be appropriate to isolate the suspension system from other components of the fluid pressure system while in float mode.

While the vent valve remains open, the tag axle will continue in the float mode. When the valve is closed, pressure is restored to the suspension, and the suspension becomes active and load bearing again. When the floating mode is actuated by a transient overload condition, the valve may be adapted to close automatically, as when the vehicle has passed beyond the road or job site condition which produced the overload. When the floating mode is actuated by the operator, the valve will remain open until reset by the operator or, if desired, may be reset automatically. The automatic reset may be controlled by a timer, by a load sensing transducer, or other condition- responsive sensing and signalling means.

As those of ordinary skill in the art will understand, the overload conditions which will actuate the floating mode will not often be encountered in highway or roadway operations. If they do occur, however, the automatic reset from floating mode to active mode may be of considerable importance.

When reset, the time for full activation of the suspension is dependent on the fluid pressure system and the speed with which it can fully re-pressurize the active suspension elements.

It is preferred that the suspension system be mounted to a portion of the tag axle assembly which is rigidly mounted to the vehicle frame, i.e., a frame extension. Such an arrangement, unlike the system defined in prior patent 5,018,593, serves to transmit shock loadings more nearly directly from the tires, wheels and axles to the frame, and minimizes the loading transmitted through the

-12- link arms. The entire assembly can be made substantially lighter and more durable in this fashion.

The lifting mechanism can be any of the commonly employed systems known to those of ordinary skill in the art. Pneumatic systems are preferred, because the preferred suspension system is pneumatic, and it is more convenient to employ the same approach. Hydraulic lift systems are equally effective, including that defined in prior patent 5,018,593. While the floating mode may be implemented with mechanical suspension systems, such systems are not preferred because of the complexity, expense and weight required to deactivate the suspension to achieve the floating mode. When the vehicle is in the back-up mode, the tag axle must be in the lifted inactive position so that undo stress or forces do not damage the tag axle assembly.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 is a profile view in schematic form of a truck vehicle (10) with a front axle tire (11), a first tandem axle set (12), a second tandem axle set (13), a tag axle (8) and a tag axle tire (14), with tag axle (8) in an active extended position in contact with a flat road surface (9) at points A, B, C, and D. In this mode, tag axle (8) is in a first, extended load bearing position, supporting a portion of the gross vehicle weight. Deployment of tag axle (8) also transfers a greater proportion of the load to the front axle. A vertically disposed plate (18) of a tag axle assembly (15) is fastened to a rear transverse box beam (16) of vehicle

(10) . When in the extended position, tag axle (8) bears a substantial portion of the weight bearing capacity of vehicle (10) . Tag axle (8) is deployed in the extended position when the vehicle is loaded to a maximum or high gross vehicle weight requiring load-sharing by tag axle (8) .

-13- Figure 2 is a view of vehicle (10) on flat road surface (9) with tag axle (8) in a second, retracted or lift position with tag axle tire (14) above road surface (9) . This position can be manually selected when: 1) the gross vehicle weight is substantially reduced, not requiring load support and it is desirable to have tag axle (8) in the elevated or lift position; and 2) the maximum gross weight is met or exceeded but the highway and bridge load-limitation laws and regulations do not apply, such as in off-road travel. The lift position is automatically put into operation when the vehicle transmission is in reverse gear, with placement in reverse gear closing a switch conveniently connected to a back-up light circuit, that automatically causes actuation of a lift bag (24) and venting of an extension bag (26), thus elevating and protecting tag axle (8) and the vehicle transmission.

Figure 3 is a view of vehicle (10) crossing an off road swale causing bridging with only front axle tire (11) and tag axle tire (14) in contact with the ground surface at points A and D. This is a very dangerous position for vehicle (10) to be in when carrying a load because of damage that can result to the vehicle frame and tag axle assembly (15). Center of gravity is located over tire (12) with vertical line E representing a vector at the center of gravity. No load is borne by the tandem axle set (12) and (13) at points B and C. It is under these conditions that the float or suspension mode will be actuated as shown in Figure 4, thus avoiding or limiting the condition shown in Figure 3. In previously designed tag axle assemblies, in a maximally loaded vehicle, a dynamic inertial loading far in excess of the design requirements for such equipment can occur with the front axle and the tag axle (8) bearing the load. This can result in either failure of the tag axle assembly (15) or failure of the vehicle frame because of unsupported excessive weight at the center of gravity, due to bridging

-14- between the front axle and the tag axle (8) . In the present invention, there is a limit switch sensor means which senses such overload and automatically overrides an extended position of tag axle (8), allowing pressure from both lift bag (24) and suspension bag (26) to bleed through an escape valve, thus placing the tag axle (8) in the float or suspension mode condition.

In the event that the switch sensor means fails to operate, there is a back-up system comprising shear bolts (19) of a vertically disposed plate (18) of tag axle assembly (15) forming a shear plane with rear transverse box beam (16) . Bolts (19) break when overloaded, resulting in separation of plate (18) from rear box beam (16), thus shifting the load support to first and second tandem axle sets (12) and (13) and avoiding damage to both tag axle assembly (15) and the vehicle frame. As those of ordinary skill in the art will readily understand, the shear plane is intended to provide an ultimate failure mode to protect from unusual and extreme circumstances, including a failure of the float mode or a bridging condition between front axle (11) and tag axle (8) even when in the lift position, as illustrated in Figure 2 or when operating in the float mode with tag axle (8) fully deflected. If such a condition is encountered, the damage may be limited to shear bolts (19) , and repairs effected by their replacement. In a still more extreme condition, shear failure may damage pneumatic lines and/or electrical wiring associated with the tag axle assembly (15), which are readily and inexpensively replaced, and may damage the delivery chute assembly mounted above tag axle assembly (15), which is also reasonably simple and inexpensive to replace, in whole or in part. Damage to the vehicle frame, to other major vehicle components, or to the mixer drum assembly are not likely to occur under any reasonably likely circumstances. Damage to tag axle assembly (15) itself will be minor or non-existent.

-15- The number, dimensions and materials of shear bolts (19) are selected to break when a shear force or weight is imposed on the shear plate joint is less than that which will cause failure of the tag axle assembly (15) or the vehicle frame, preferably with a reasonable margin of safety. Engineering the shear bolts (19) requirement is well within the level of skill in the art.

Figure 4 is a view of vehicle (10) over an off-road surface area that can range from flat to irregular. Tag axle (8) is in an active float mode (14A) with tag axle tire (14) able to react to road surface topography. In the float mode, the trailing link arm (20) is allowed to pivot freely on pivot pin (22), with the tires in contact with the ground. With the float mode active, no load is transmitted from the vehicle to the axle, wheels and tires. Tag axle (8) is not in an active or weight bearing mode but can react to changing road surface conditions. The float mode, in reaction to ground topography, provides a damage-preventing safety mechanism which protects the vehicle frame and tag axle assembly (15) from a damaging overload. As described above, the tag axle (8) can be placed in the float mode either manually by the driver or through an automatic switch sensor means that opens both lift bag (24) and extension bag (26) to escape valves thus reducing air pressure in the bags to ambient air and allowing tag axle (8) to passively react to ground surface irregularities. The inactive suspension system permits tag axle (8) to freely move upwardly and downwardly, as illustrated in Figure 4, where wheel set (14) is shown in a downward position and in an upward position (14A) shown in the phantom line representation. Only the unsprung weight of tag axle (8) tag axle wheel set (14) , and link arm (20) are borne by tag axle (8) in such a configuration, and the vehicle weight is transferred forward to tandem axle sets (12) and (13) .

Figure 5 is a schematic detailed drawing of tag axle assembly (15) in the lift position. A plurality of shear

-16- bolts (19) fasten plate (18) to rear box beam (16) creating a shear plane. A trailing link arm (20) is pivotally connected at a proximal first end to plate (18) with a pivot pin (22) allowing rotation of link arm (20). Link arm (20) is distally connected at a second end to an axle and wheel assembly (23). Pneumatic lift bag (24) is located on a rear member (25) of plate (18), lift bag (24) fastened to link arm (20) . Actuation of lift bag (24) displaces link arm (20) pivotally around pivot pin (22) causing elevation of tag axle (8) . Load bearing pneumatic extension bag (26) is connected to the underside and rear¬ most area of plate (18), extension bag (26) additionally connected to the top surface of link arm (20) . Actuation of extension bag (26) extends tag axle (8) by bringing pressure against the top surface of link arm (20) as shown in Figure 6. Lift bag (24) and extension bag (26) are complimentary, with air pressure increasing in one simultaneous with pressure reduction in the other. An air-pump pressure means, not shown, forces air into one while venting means allows it to escape from the other. The air-pump means and venting can be controlled manually by the driver with a lever control means, or it is controlled automatically by a switch means when the vehicle transmission gear box is placed into reverse. The limit switch may be conveniently activated through a back¬ up light electrical circuit. There is also a sensor switch means that senses tag axle overload and automatically de-pressurizes extension bag (26) through the extension bag escape valve thus placing tag axle (8) in a passive or float mode. Tag axle (8) may be controlled either manually or automatically by either a hydraulic, electrical, pneumatic or a manual suspension relief means.

Figure 6 schematically illustrates tag axle (8) in the extended, load bearing position with actuation of extension bag (26). In this embodiment, extension bag (26) is actuated through a pneumatic air-pump pressure

-17- means. Tag axle (8) may be extended in an active mode in which extension bag (26) is fully pressurized or in a float, de-pressurized mode in which the escape valves of both lift bag (24) and extension bag (26) are open, thus allowing tag axle (8) to passively react to ground topography. The means for placing tag axle (8) in the float mode have been described above.

As will be familiar to those of ordinary skill in the art, tag axle (8) of the present invention will preferably be equipped with brakes, shock absorbers and self steering, such as camber self steering. Although such preferences are not requirements of the invention, they are common features employed in the art and provide known advantages. Figure 7 is a schematic representation of the pneumatic circuits which pressurize and vent the air bags, and the contact system which actuates the pneumatic components.

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Claims

CLAIMSWhat is claimed is:

1. A vehicle having a lift axle to bear a portion of the vehicle gross weight in highly loaded conditions, said lift axle being carried on a lift axle assembly comprising:

(a) a pivotally mounted link means for carrying and locating said axle in a first, extended ground contact position and a second, retracted lift position, said lift position carrying all elements of said lift axle out of contact with the surface on which the vehicle is borne;

(b) an axle actuator means, said means extending said axle to said first position and lifting said axle to said second position; (c) a control means to operate said actuator means;

(d) a suspension means for selectively transmitting a portion of said vehicle weight to said lift axle in said first, ground contact position, said suspension means having an active load bearing mode and an inactive non-load bearing mode while said lift axle is extended to said first position and being inactive while said lift axle is retracted to said second position; and

(e) a suspension actuator means, responsive to a control signal while said lift axle is extended to said first ground contact position, for activating and deactivating said suspension means.

2. The lift axle of claim 1, wherein said suspension means is normally active when said lift axle is in said first, ground contact position and is deactivated in response to said control signal.

3. The lift axle of claim 2, wherein said control signal is responsive to an overload condition imposed on said lift axle.

4. A lift axle according to claim 1, wherein said axle actuator means is pneumatically activated air bag.

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5. A lift axle according to claim 1, wherein said axle actuator means is hydraulically controlled.

6. A lift axle according to claim 1, wherein said axle actuator means is electrically controlled.

7. A vehicle having a rear-mounted lift axle to bear a portion of the vehicle gross weight in highly loaded conditions, comprising:

(a) a rear transverse horizontal frame member on said vehicle, said frame member having a generally rearward facing, vertically disposed frame plate;

(b) a lift axle assembly provided with a generally forward facing, vertically disposed axle assembly plate adapted to mount to said frame plate to position and locate said lift axle assembly; and (c) fastening means for fastening said axle assembly plate to said frame plate and for transmitting a portion of said vehicle weight to said lift axle assembly, constituting a shear plate joint, said fastening means having a shear failure strength greater than acceptable loads on said lift axle assembly and said vehicle and less than loads which are capable of damaging said lift axle assembly or said vehicle.

8. The vehicle of claim 7, wherein said shear plate joint fails under loads greater than about 90% of the shock loading which is capable of damaging said lift axle assembly or said vehicle.

9. The vehicle of claim 7, wherein shear failure of said shear plate joint causes said lift axle assembly to be displaced in a generally vertical direction to avoid substantial damage to said lift axle assembly and said vehicle.

10. The vehicle of claim 7, wherein said shear plate joint is located rearward of and at least as high as said truck frame.

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