There are several hydraulic shock absorber designs in use today for commercial vehicles:
BASIC TWIN TUBE DESIGN
The twin tube design has an inner tube known as the working or pressure tube and an outer tube known as the reserve tube. The outer tube is used to store excess hydraulic fluid.
There are many types of shock absorber mounts. Most use rubber bushings between the shock absorber and frame or suspension to reduce transmitted road noise and suspension vibration. The rubber bushings are flexible to allow movement during suspension travel. The upper mount of the shock absorber connects to the vehicle frame.
Notice that the piston rod passes through a rod guide and seal at the upper end of the pressure tube. The rod guide keeps the rod in line with the pressure tube and allows the piston to move freely inside. The seal keeps the hydraulic oil inside and helps exclude contaminants. The base valve located at the bottom of the pressure tube is called the compression valve. It controls fluid movement during the compression cycle.
When the shock absorber is in a compression stroke, some oil in the lower working chamber transfers across the piston via the lightly loaded inlet valve. The remaining oil (corresponding to the volume of the piston rod entering the working chamber tube) is forced through a valve system in the base valve and then passes into the reserve tube or equalization chamber. The velocity of rod movement and the valving in the base valve determine the resistive force generated by the shock absorber on compression.
When the shock absorber is in rebound stroke, the inlet valve in the piston closes and the oil in the upper working chamber is forced through a valve system in the piston.
To compensate for the rod volume that is leaving the inner tube, oil passes from the outer oil reservoir via a lightly loaded inlet valve in the base valve into the lower working chamber, thus keeping the inner tube full at all times. Rod velocity and piston valving determine the resistive force generated by the shock absorber on rebound stroke. Shock absorbers also have a drainage orifice below the seal that allows oil to drain into the reserve tube. This also allows air to escape from the pressure tube, which helps prevent fluid “foaming.”
Bore size is the diameter of the piston and the inside of the pressure tube. Generally, the larger the unit, the higher the potential control levels because of the larger piston displacement and pressure areas. The larger the piston area, the lower the internal operating pressure and temperatures. This provides higher damping capabilities.
Ride engineers select valve codes or damping force values for a particular vehicle to achieve optimal balance and stability under a variety of driving conditions. Their selection of bleeds, deflective valve discs, springs and orifices helps control fluid flow within the unit, which ultimately determines the feel and handling of the vehicle.