What is Double Acting Hydraulic Cylinder | Construction, Working

Definition

When oil acts on both side of piston i.e. on face side for forward movement and piston rod side for return movement. It is known as double acting hydraulic cylinder.

Construction

• Cylinder body – It is seamless tube of Al alloy or steel with chrome plated to provide better corrosion and scratch resistance, stainless steel, cast iron also used for body.
• Piston and piston rod – The pistons are made of aluminium alloy castings. They are fitted precisely in the cylinder bore with the help of piston seals. Piston rod is attached to the piston and made of steel bar.
• Piston seals – It is provided for leak proof assembly between cylinder bore and piston.
• Bushing – Bush are provided for smooth movement of piston rod and for reducing leakage of oil. They are made of brass, bronze, babbit metal and press fitted to cap end.
• End covers :- The end covers are provided at the front end as well as at rear end.

Working

• When pressurised oil enters through inlet A from face side of piston, it pushes the piston to the right hand side to complete forward stroke
• while the oil on the rod side is delivered through the outlet to the tank through the outlet B.
• When pressurised oil enters through inlet B from piston rod side. It moves the piston to the left hand side and completes the return stroke.
• The oil from the face side is drained out through outlet A to the tank.
• It gives to and fro (reciprocation) movement of piston.

Cushioning of double acting cylinder

• Cushioning of cylinders means gradual deceleration of the piston near the end of its stroke. It means slowing down the speed of the piston near the end of cylinder body.
• It is very helpful to reduce shock or impact of load on the cylinder end covers specially when a heavy load is connected to the rod or the cylinder and is working at very high speed.
• The cushion assembly is around 25mm long for a standard cylinder. It consists of a small passage to allow entrapped oil to the port with a cushion needle with an check valve to allow free flow of oil during reverse flow of oil during reverse start of piston travel.
• The end of the cushion nose is tapered in order to enter more easily into the cushion chamber.
• The fluid is normally leaves through the outlet port directly, but when cushioning boss enters the recess, the fluid around the piston is trapped.
• The only way the fluid can escape is through the secondary path which is restricted by needle valve.
• The needle valve is adjusted so that the piston is slowed up over the last part of its stroke by a pressure build up in the fluid escaping past the needle valve. 

Symbol

Applications

• It is used in machine tool like shaper machine, table movement of milling, hydraulic press machines etc.
• It is used in material Handling equipment, construction equipment, etc.
• It can be used for push and pull force.

FAQ

• How many inlets to double acting cylinder ? 
  A Double acting hydraulic cylinder has two oil inlets for hydraulic oil at both the ends of the chambers.
   On passing hydraulic oil from one end, piston moves forward and pushes the oil out from the other end and vice versa. 
  So each end of the cylinder acts as both inlet and outlet for the hydraulic oil.

• What is a double acting hydraulic cylinder?
  When oil acts on both side of piston i.e. on face side for forward movement and piston rod side for return movement. It is known as double acting cylinder.
• Where double acting cylinder used for?
  Image result for double acting hydraulic cylinder
  Double-acting cylinders are typically used in all applications where the thrusts and stroke lengths required are in excess of those available from single-acting cylinders.
• Why Is Cushioning Necessary?
  Cushioning is needed to lower the speed of the cylinder before it reaches the end cap. Lowering the speed of the piston helps reduce stress on the components within the cylinder. It also lessens vibration conveyed to the other parts of the machine.
• How does a cushioned hydraulic cylinder work?
  How do hydraulic cylinder cushioning decelerate cylinder? As mentioned, the cylinder retracts when the fluid flows outwards and when the piston rod approaches the end cap. In a cushioned hydraulic cylinder, the cushion plunger will enter into the cushion cavity when it approaches the end cap.