Automotive transmission and braking systems are mechanically complicated devices but they have been refined over many years and tend to work well. However, many of the refinements that make cars easy to drive have not been successfully adapted to big trucks due to power and torque requirements. These include disk brakes, automatic transmission, manual transmission synchronizers, limited slip differential and independent rear suspension.
Disk brakes have been tried on big trucks but even vented disks cannot dissipate heat as fast as drums and drum brakes are barely sufficient for the task. Drivers must be careful to not overheat the drums by limiting downhill speed and assisting the friction brakes when possible with an engine compression brake on steep or long downgrades. Overheated brakes is a common occurrence for big trucks and is very dangerous due to the possibility of brake failure or fire. Even if no accident or fire results from an overheating, the drums will distort from round and the brake pads will cool in a glazed state requiring the replacement of all brake shoes and drums on the vehicle.
It is not uncommon for brake failure or damage to occur if a drive has to make a fast stop from highway speed or on a downgrade from a speed of only 35 MPH. When drums brakes are overheated, white hot metal is thrown from the drum in all directions between the wheels and it can start a fire in the tractor or trailer floor. From a speed of 60 MPH, brakes can heat to white hot in only a few seconds. For this and other reasons, drivers are required to carry a fire extinguisher and to check the brakes frequently especially before and after long downgrades.
Automatic transmissions have been developed for busses but thus far have been considered a failure for big trucks. They are larger and heavier than manual transmissions and are less reliable. This results in high operating costs due to additional fuel usage, service failure and maintenance. Most large trucking companies have no big trucks with automatic transmission. Some smaller companies and independent owner-operators use automatic transmissions to accommodate drivers having difficulty with manual transmissions. But many potential truck drivers are excluded from the profession due of the longer reach required to disengage the clutch and the additional force required to operate the pedals.
However even experienced truck drivers will have problems with the manual transmissions in some driving situations. The clutch pedal on big trucks requires considerably more force to operate than auto clutches. Although not usually a problem in long distance driving, city driving and other stop-and-go situations can cause fatigue and possible stress related injuries to the driver’s left leg. Drivers ore often walking in mud, snow, water and oil and parking accidents sometimes occur because the driver’s foot slips off the clutch pedal.
Big truck transmissions remain difficult to shift in ideal conditions but are nearly impossible to shift easily in some conditions. Without synchronizers drivers are taught to double-clutch while shifting gears. Ten speed transmissions use two or three countershafts whereas auto transmissions need only one. The heavy countershafts combined with the large diameter clutch plate gives the transmission input considerably more angular momentum than a car transmission. In normal situations double clutching provides the means to change the input shaft speed using the engine. However when shifting between lower gears in a steep hill climb or descent double clutching does not work since the vehicle gains or loses speed as fast as the engine. Also, when releasing the clutch under heavy load, the spring loaded clutch plate will resonate against the input shaft and prevent engagement of the next gear. Drivers then must force the transmission quickly between low gears and hope that nothing breaks.
Big trucks have gears sufficiently low to enable climbing steep hills, however, uneven shifting contributes to freight damage as pallets of freight can rock back and forth sometimes causing the pallets to collapse. On steep upgrades this rocking action can combine with the incline to cause freight to break loose from its bindings and slide to the rear of trailer sometimes damaging or even breaking open the trailer doors.
Although trucks move slower than cars with the engine idling in reverse or low gear, the speed is still too fast for docking and parking maneuvers. When backing on a flat or uphill driver must “feather the clutch” to limit speed to only 1 or 2 MPH. When backing on a downgrade it is usually not possible to “feather the brake” to limit speed due to the delay in response of the air brakes. A driver can limit speed by rolling back short distances with intermittent stops but this sometimes exhausts the air pressure and the driver may have to wait for the vehicle air pressure to build before continuing. Another technique often used is to hold brake pressure sufficient to stop the vehicle while “feathering the clutch” to overcome the brakes. Although this tends to work, it is hard on both the brakes and the clutch.
For large trucking companies, maintenance costs are second only to fuel costs and considerably higher than driver commissions. Broken drive train and braking components are a significant portion of the high maintenance costs. The potential exists to significantly reduce these maintenance costs with an alternative fluid drive and braking system and a microprocessor controlled or “Smart” system would be immune to damage due to driver abuse.
With eight driven tires on a big rig tractor, one might not expect trucks to become easily stuck and need assistance from a tow truck but this is also a common occurrence. Most tractors are equipped with an inter-axle differential (also known as a power divider) lock but not an intra-axle lock or limited slip differential. The power divider lock assures that the vehicle is stuck only if a set of dual wheels is unweighted on both driven axles, however, this can occur in several ways.
Drivers often have to drive in mud lots and try to pick up a trailer that might be stuck in mud and potholes tend to form where the drive tires will be in front of trailer parking spaces. A single deep pothole can cause the vehicle weight to shift to two diagonally opposite dual wheels. Since a set of wheels is unweighted on both axles, the power divider lock does not help.
Other situations will unweight all driven wheels on one side of the tractor. If the trailer was parked on uneven ground or the wheels have sank into the ground on only one side then the trailer can tilt the tractor enough to unweight the drive wheels on one side of the tractor. Many customer sites require the driver to park the trailer on an upgrade or downgrade while maneuvering the tractor on level ground. If the grade is steep enough, then the trailer will unweight the drive wheels on one side of the tractor and again it will become stuck. These situations are made worse by the very significant torque that the drive system puts on the driven axles when in low gear or reverse. Even turning at low speed on level ground can cause wheels to tend to slip on one side of the tractor.
Aside from the operability problems with big truck drive systems is the size, weight and cost of the components. Most auto drive system components can be carried by hand but most big truck drive components require a crane or other machine to move. A ten speed transmission measures approximately 1 foot deep by 3 feet wide and 4 feet in length. The drive shaft is about 6 inches in diameter and the differential housings are nearly 2 feet in diameter.
Weight of the tractor is important to trucking companies not only because the extra weight reduces fuel economy and increases maintenance for every tractor mile, but because big trucks have a gross weight limit of 80,000 pounds. Any weight that can be removed from the tractor increases the payload capacity of the trailer. Currently shippers must be careful not to put too much weight in the front of the trailer. If the weight on the drive axles exceeds 34.000 pounds the the driver must return to the shipper from the nearest weigh station to have the load reworked. This is very expensive and often will cause a late delivery.
Development of a powerful wheel motor and “Smart Axle” anti-skid control may have additional application on non-powered axles for hydraulic braking. Trailer tandems carry the same 34,000 pound maximum weight as the tractor driven tandems and have the same requirements for braking when the trailer is loaded to maximum weight. Current trailer friction brakes are designed to work best for a loaded trailer and do not compensate for an empty or lightly loaded trailer. This causes the trailer wheels to tend to lock up before the tractor wheels with the potential to cause a trailer jackknife.
Many trailers are equipped with an anti-lock braking system to help prevent trailer jackknife but the systems do not work well. Current pneumatic anti-lock braking systems tend to be unreliable and create other problems when they are functional. Pneumatic control has a slower response than equivalent hydraulic control and this can cause resonance interfering with the tractor ABS and significantly increase the vehicle stopping distance. Since increased stopping distance is often more dangerous than a trailer jackknife, many drivers will disable the trailer ABS and trucking companies are reluctant to maintain the trailer ABS.
The risk of a vehicle fire caused by overheated brakes is greater for the trailer tandem than for the tractor tandem. The trailer floor above the trailer tandem is usually wood while the trailer floor above above the driven tandems is usually steel plate. Being closer, the driver is more likely to see or smell overheated brakes on the drive tandem. Depending on the value of the cargo, a trailer fire can be more costly than a tractor fire. This is especially the case if the cargo is hazardous.
Hydraulic motor braking could significantly improve trailer brake response and dissipate heat from oil sheer safely. Although it may be cost prohibitive to retrofit a large trailer fleet with hydraulically braked tandems, trucking companies carrying high value loads and hazardous cargo would likely be interested in the safest design for their trailers.
Alternate Vehicle Drive Systems
Most of the above mentioned operability problems would be solved by a transmission system that could drive all wheels or dual wheels independently such that some torque would be available to the driver even one driven wheel has contact with the ground. If a system could be contained within the axles, then the potential exists for a tremendous reduction in the cost, size and weight of the vehicle drive system.
Many commercial vehicles use alternative drive system based on electric motors or hydrostatic motors and pumps having a continuously variable displacement. These vehicles include earth moving equipment, heavy movers, off-road motorcycles and experimental cars. Hydrostatic motors have the highest power to weight and size ratio of any motor type and are generally used on the drive axles and often located within the wheel hub. Electric motors may provide the primary drive for lighter vehicles but often are used in large vehicles as an assist for steer wheels.
Although some large hydrostatic motor driven vehicles can reach highway speeds, they cannot do it efficiently. At higher speeds much energy goes into maintaining a rapid flow of hydraulic fluid. Several hydrostatic motor designs can be scaled in size to meet the torque and power required to directly drive the wheels or dual wheels of a big rig tractor. The challenge then is to increase the efficiency to an acceptable level for a highway vehicle. A continuously variable transmission system could improve engine efficiency by allowing it to always run at the optimum speed for the required output torque and by eliminating the engine acceleration and deceleration required for double clutch gear shifting. Even so the new transmission system would probably need better than a 95% efficiency. There are some recent patents for variable eccentricity motor designs claiming to have achieved better than 95% efficiency at speeds up to 1500 RPM.
A continuously variable transmission could be contained within a straight axle using three variable displacement hydrostatic motors provided the displacement can be reversed such that any motor can become a pump for either direction of rotation. Most variable displacement motor designs have this flexibility. The axle transmission system would replace the differential and use one motor/pump for connection to the engine and one for each wheel or dual wheel. The wheel speed on a big truck is 523 RPM at 70 MPH while the optimum diesel engine speed is typically around 1800 RPM. Due to this difference in speed the optimum engine motor/pump design may be different from the wheel motor/pump design.
Although the engine motor/pump would normally run as a pump, i.e., a positive displacement, it would also need to reverse to drive the engine faster for engine compression braking. However a hydrostatic motor drive system could provide another emergency braking system when the engine brake is insufficient and the friction brakes fail due to overheating. Energy could be converted to heat by oil sheer through an over-pressure relief valve and then dissipated through an oil cooler.
Statement of Need