The present invention relates to an air spring apparatus for a vehicle suspension, and more particularly to such an apparatus in which the spring rate of the air spring is variable according to the dynamic conditions encountered.
2. Description of the Prior Art
In vehicles such as large trucks, a properly designed suspension will maintain a relatively constant distance between the axle and vehicle frame under all conditions of speed, load and road profile, and it will also act as a vibration isolator or dynamic dampener to absorb road shocks. Ideally, actual applied loads transferred to the road through the wheel should not exceed ten percent of the gross axle load in a static condition, that is, with the vehicle standing still. When loads exceed this limit there is a potential for damage to the road and to the vehicle, which becomes a reality at high levels of load, vehicle speed, and road irregularity. Despite various attempts to maintain applied loads to an acceptable level, the nation's highways have suffered severe and continuing damage, and many believe a large part of the blame for this is the inability of planners to recognize that improper vehicle suspensions are the major controllable culprit.
Typically, a truck suspension takes the form of a bridge beam, as in a leaf spring suspension, or a cantilevered beam, as in an air bag suspension. No matter what its form, the shock isolator or dampener operates like a spring, ideally with a very low spring constant. However, this ideal cannot be met because there is a constricted space adjacent the wheel, and between the wheel and the frame, and such a low spring rate spring would not fit into the available space. Further, a high rate spring is needed to maintain the desired frame height above the axle under maximum load conditions.
The load may vary, for example, between 2,000 to 20,000 pounds per axle, more than half the time falling within the range of 3,000 to 8,000 pounds. Provision must be made for some overloading, even though perhaps not authorized by the carrier, and also for dynamic overloading.
Assuming the shock isolator is a leaf spring, which is common in the prior art, effective shock isolation is only possible at or very close to maximum allowable load. The spring constant, which is necessarily high to support the maximum load, does not change for other loads. Under partial or no load conditions, the high spring rate of the leaf spring does not permit the spring to be significantly deflected, and there is inadequate absorption of the energy resulting from wheel impact against road obstructions. Consequently, at partial loads the lead spring acts essentially like a solid member, and all road shocks are passed without attenuation to the sprung mass of the vehicle. This essentially direct transmission of road shocks may take place at the natural frequency of the sprung mass, in which case the resultant oscillation of the vehicle and suspension system can render handling of the vehicle difficult, and subject both the vehicle and the road bed to constant pounding.
It is possible to modify the unsatisfactory results flowing from a high spring rate suspension by supplementing the leaf spring beam with a secondary spring in series with the leaf spring beam. However, this is effective only at no-load, and poor shock isolation then exists between the no-load and maximum load conditions.
In my U.S. Pat. No. 3,920,264, issued Nov. 18, 1975 for "Vehicle Low-Load Isolator Spring Suspension Apparatus", an isolator spring means serves as an auxiliary spring for interposition between the usual leaf spring and the sprung mass of the vehicle to handle suspension under no or low-load conditions. The auxiliary spring completely deflects or bottoms out under heavy load conditions whereby the leaf spring then assumes the vibration energy absorption function. In effect, the auxiliary spring acts as a "soft" spring located between the "hard" leaf spring, which is mounted between the frame and the wheels, axles and associated parts constituting the unsprung mass. Such an auxiliary spring system works reasonably well, but insufficient room exists adjacent the wheel and leaf spring suspension area to accommodate the size of auxiliary spring system capable of handling more than a narrow range of loads. Moreover, to be completely adaptable to the many different truck and suspension designs, a proper apparatus of this type would require auxiliary coil springs of different spring rates, which is a practical impossibility.
Various other means have been tried to provide adequate vibration isolation, but without particular success. For example, pneumatic or air bags have been used in combination with conventional shock absorber cylinders. Such bags are located between the vehicle frame and axle, with the axle held in position by a cantilevered beam pivoted at the frame. Air is pumped into each bag from the truck supply until the axle is located at the desired level relative to the frame.
The material of which the bag is made is elastic. When it is placed under a load, the air in the bag is compressed and the bag distends at a rate proportional to the elasticity of the bag. The air bag does not act as a true air spring in that it is not responsive to Boyles gas laws, that is, its volume does not change in direct proportion to pressure changes. Consequently, although the spring rate varies somewhat over its range of operation, it is essentially constant regardless of the load being supported. It is influenced primarily by the elasticity of the material of which the bag is made, that is, such elasticity largely determines the spring rate.
Inasmuch as the spring rate of such an air bag is a constant, it is therefore no more capable of accommodating a range of loads than was the auxiliary or secondary coil spring arrangement.
On inflation of such an air bag, the surfaces between which it is disposed are separated a predetermined amount at some predetermined internal air pressure. When the vehicle load increases, the surfaces tend to move together, increasing the forces on the bag and the bag expanding it a distance directly proportional to the elasticity of the material.
Typically, the spring rate or elasticity of the air bag material is selected such that it is low enough to operate effectively as a shock isolator for the usual frequencies developed by the wheel/road profile contact. This low rate is productive of a natural frequency in the same range as the frequencies transmitted to each bag by the associated vehicle wheel. Consequently, order to avoid resonance and destruction of the bag it is essential that the bag operate in parallel with a secondary, two way damping mechanism such as a double acting shock absorber.
Air bag systems not only suffer from the fact that such secondary shock absorbers must be used in conjunction with the air bags, but air bags are also relatively costly to purchase and maintain. Further, they are heavy and bulky, too bulky to be used in conjunction with vehicle steering axles. In addition, an air bag system can only be used with a cantilevered beam, and over three-quarters of the heavy duty trucks on the roads today use some form of spring beam such as a leaf spring.
What is needed is a road shock or vibration isolator of compact form, adapted to fit in the constricted space available, and capable of automatically adjusting its spring rate to provide optimum isolation of impulsive forces at any point of load within its design limits.