This invention relates to a suspension system for use on a vehicle and is installed between an axle and a load carrying body.
Suspension struts are used to support the load bearing portion of a truck from its axle. One important strut system uses a gas over oil. The gas is highly compressible and the oil is essentially non-compressible. An outer tube is fixed or connected to the body or frame of the truck. Positioned inside the outer tube is a hollow piston rod. An annular space is provided between the outer tube and the piston rod. A plunger tube, which is connected to the axle of the truck, extends upwardly into this annular space between the piston rod and the outer tube. An oil is injected into the bottom portion of the main cavity within the piston rod and flows through ports in the lower end of the piston rod to an annular cavity between the piston rod and the inner wall of the plunger tube. A check valve also permits flow from its main chamber into the annular cavity but stops the flow in the opposite direction. A gas which is normally nitrogen is injected under pressure above the oil. This gas/oil combination can be called a gas spring.
As the outer tube moves down with respect to the axle due to heavy loads being carried by the vehicle, the volume of the main cavity decreases thus forcing oil out through the port and into the annular cavity and the gas is compressed to a rather high degree.
It is important for the oil to flow as free as possible into the annular cavity to prevent cavitation or a negative pressure on the oil. The primary problem of the negative pressure is the seal's inability to handle this reverse pressure. The result is air and particulate contamination are allowed to enter the normally closed environment. The air can radically change the operating characteristic of the system while the particulate contamination can shorten the operating life.
When the load on the suspension decreases, the outer tube moves up with respect to the plunger tube forcing oil to flow from the annular cavity only through the port means into the main cavity. This can be called the rebound stroke of the suspension. This rebound stroke is dampened by the restriction created by the port means to prevent rapid cycling of the suspension system.
It is virtually impossible to calculate the size port which will give the proper operating functions for the suspension system to obtain the optimum damping action. Therefore, typically what is done is to assemble the suspension with a selected port size. The truck is then operated under a specific set of load conditions. The suspension is then disassembled and reassembled with a different port size. The truck is again operated under the same load conditions. This is repeated until an optimum port size is found for the tested load condition. However, due to changes in operating parameters, the operating conditions change. It is very impractical to change or tune the porting with these changing operating conditions. Therefore, optimum damping is lost.
Proper alignment between the piston rod and the plunger tube is very important. There are bushings or bearings between the outer tube and the plunger tube and these must be properly sized.