Hydraulic cylinders are the most common type of hydraulic actuator. They are widely used throughout almost every hydraulic machine for moving, lifting, clamping or steering etc.
Look at every excavator working on the road works near you and you will see several large cylinders lifting and lowering the each of the buckets
Apply pressure to the rod of a single acting or plunger cylinder, and the rod will extend unless the mechanical load on end of the cylinder is sufficient to push it back and retract the cylinder.
A double acting cylinder has will extend with pressure on the rod end but also retract with pressure on the annulus side. Seals prevent leakage across the piston head and rod seals stop leakage passed the rod to atmosphere.
Cushions work by slowing the movement as the cylinder reaches the end of its stroke. Our diagram show how it does this by trapping fluid in a small pocket and only releasing it through a small orifice. This fluid pocket will need a check valve to stop negative pressures being generated as the cylinder annulus opens.
Single acting cylinders only allow pressure on one side to extend them.
Double acting cylinders are by far the most widely used style of cylinder. They are also available in a double rod design which has no full bore are side, but instead rods on both sides so the pressure areas remain the same. This is beneficial when loads need to be matched or when running high accuracy control systems where a finely balanced performance is required for both retracting and extending.
Telescopic cylinders are available which contain a number of extending sections allowing them to extend to over two or three times their retracted length.
Cylinders come with a wide range of mounting styles. Suitable mounting is critical for avoiding side loads on the cylinders as well as the risk of the buckling the rods. These include Clevis plain or self-aligning - trunnion - flange - foot
Cylinders also generally fit into one of three main groups. Welded cylinders offer the best value but lowest pressure and shortest working life. Tie rod cylinders have better performance characteristics and are generally used in industrial rather than mobile applications. Finally, Mill Cylinders have the highest performance and durability ratings.
Other options that are available include cushions, to smooth the rod deceleration, and bleed nipples to vent out air while commissioning. position feedback is available built into the cylinder as are many different seal types including gland drain between seals.