1. Hydraulic Cylinders • Hydraulic Motors
Chapter 7
Hydraulic Actuators
Hydraulic Cylinders • Hydraulic Motors

2. Hydraulic cylinders are categorized as ram, single-acting, and double-acting.
Hydraulic cylinders can be mounted in hydraulic systems using different mounting methods. The mounting method used depends on the needs of the hydraulic system and the type of hydraulic cylinder. The different types of hydraulic cylinders are categorized as ram, single-acting, and double-acting. See Figure 7-1.

3. A ram cylinder has the simplest design of all hydraulic cylinders because it has only one chamber and provides force in only one direction.
A ram cylinder is a single-acting cylinder that has a large piston and a rod with the same diameter as the piston, and is capable of producing large amounts of linear force during extension. A ram cylinder consists of a cylinder barrel, a cap-end end cap and port, a piston and rod, a ring stop, and rod seals. See Figure 7-2. It is the simplest design of all hydraulic cylinders because it has only one chamber in the cylinder barrel and provides force in only one direction. Ram cylinders are often used in applications such as plastics extrusion presses, hydraulic jacks, and metals processing equipment.

4. Most single-acting cylinders have a rod-end vent that allows air to enter and vent from the rod end of the cylinder.
Most single-acting cylinders have a rod-end vent that allows air to enter and escape from the rod end of the cylinder. See Figure 7-3. This prevents pressure buildup during extension or vacuum during retraction. When there is pressure buildup or vacuum in a hydraulic cylinder, the piston and rod require more energy to move to their desired position. Single-acting cylinders are available in several variations including single-acting, spring-return cylinders and telescoping ram cylinders.

5. A single-acting, spring-return cylinder uses fluid flow for extension and a spring for retraction.
A single-acting, spring-return cylinder is a hydraulic cylinder that uses fluid flow for extension and a spring for retraction. See Figure 7-4. The piston and rod are retracted when the cylinder is in its normal position. A single-acting, spring-return cylinder operates through the following procedure:
1. Hydraulic fluid enters the cap-end port, forcing the piston and rod to extend from the cylinder barrel.
2. As the piston and rod extend, air escapes out of the rod-end vent to prevent pressure from building up on the rod side of the piston.
3. Fluid flow to the cylinder is stopped. The cylinder is connected to the reservoir, allowing the spring to retract the piston and rod.
4. As the spring returns the piston and rod to their retracted position, air fills the rod end of the cylinder and assists with retraction.

6. A telescoping ram cylinder is a hydraulic cylinder that extends its rod in stages.
A telescoping ram cylinder is a ram cylinder that extends its rod to a length longer than the housing of the cylinder by extending it in stages. See Figure 7-5. Each stage of the rod is a set length. As hydraulic fluid enters the cylinder, each stage extends at an equal rate until the cylinder is completely extended. After the rod has extended to the required length, it retracts as hydraulic fluid flows out of the cylinder. The stages collapse together because of gravity and the weight of the load, such as the bed of a dump truck.

7. Double-acting cylinders are able to produce force in both directions by applying hydraulic fluid pressure to either side of the piston.
A double-acting cylinder is a cylinder that uses metered fluid flow for extension and retraction. Double-acting cylinders are the most common of all hydraulic cylinders. They are able to produce force in both directions by applying hydraulic fluid pressure to either side of the piston. See Figure 7-6. Double-acting cylinders are available in a variety of lengths and diameters and can be mounted using different methods.

8. A double-rod cylinder can provide equal force and piston speed in both directions.
A double-rod cylinder is a hydraulic cylinder that has a single piston and rod that protrudes from both end caps of the cylinder. See Figure 7-7. A double-rod cylinder has the same amount of surface area on both sides of the piston. This will produce the same amount of force during extension and retraction. A double-rod cylinder can also provide equal piston speed in both directions and can be more productive than a single-rod cylinder. The control of a double-rod cylinder is the same as with a double-acting cylinder.

9. Tandem cylinders consist of two or more in-line cylinder barrels with their rods connected to form a common rod.
A tandem cylinder is a hydraulic cylinder that consists of two or more in-line cylinders with their rods connected to form a common rod. See Figure 7-8. Tandem cylinders have two pistons on the same rod and have four ports. Two of the ports always work together as the inlets, while the other two ports work together as the outlets.

10. Duplex cylinders have the same assembly as tandem cylinders, but the rods are not connected.
A duplex cylinder is a hydraulic cylinder that consists of two or more in-line cylinders that do not have their rods connected to form two or more cylinders in one housing. Duplex cylinders have the same assembly as tandem cylinders, but the rods are not connected together. See Figure 7-9. Duplex cylinders can be used to accomplish and maintain different fixed stroke lengths. They are used in applications such as two-post hydraulic lifts, which are used to lift vehicles in automotive repair facilities.

11. A cushion is a tapered plug attached to the piston and rod and includes a needle valve with a check valve in the end cap at the rod end, cap end, or both.
A variable cushion is a needle flow control valve with a check valve in the rod-end end cap, cap-end end cap, or both and a tapered plug attached to the piston or rod to slow the piston at the end of its stroke. Variable cushions prevent the piston from colliding with the inside surfaces of the cylinder end caps. Variable cushions are also adjustable. See Figure A variable cushion can be used on either end cap of the cylinder and operates through the following procedure:
1. As the piston gets close to the end of its stroke, the cushion attached to the piston or rod fills the space of the exit hole.
2. As the exit hole is filled by the cushion, all remaining fluid is forced to flow through the needle valve on the same end of the cylinder.
…Complete procedural list on page 220.

12. Stop tubes prevent cylinder rods from sagging and pistons from pivoting.
Cylinders with long piston rods sometimes have stop tubes installed. A stop tube is a short hollow metal tube attached to the rod end of a hydraulic cylinder that changes the piston stroke. A piston stroke is the distance the piston and rod of the cylinder travel. For example, a cylinder with a stroke of 4 will have a piston and rod that travel 4 inside the cylinder barrel. The rod will also travel 4 from the beginning of the stroke to the end of the stroke. See Figure A long rod can sag under its own weight. A stop tube prevents the rod from sagging by moving the piston away from the end cap, which provides a length of straight rod between the piston and rod steady bearing. It also prevents the piston from pivoting and destroying the piston rings.

13. A seal creates positive contact between the components of a hydraulic cylinder to help contain pressure and prevent leaks.
A seal is a device that creates contact between the components of a hydraulic cylinder to help contain pressure and prevent leaks. See Figure If system pressure is too high, the cylinder can leak hydraulic fluid, decreasing the efficiency of the system or completely stopping the cylinder from operating. Seals are installed between the metal parts of cylinders or other hydraulic components to prevent leakage between the parts. Seals can be categorized as positive or nonpositive and static or dynamic.

14. A back-up ring is installed on the side of the O‑ring receiving the least amount of pressure or on both sides if the O-ring receives pressure from both directions.
Excessive fluid pressure can force an O‑ring out of its groove and into any space. Using back-up rings in the O-ring groove can help prevent the extrusion of the O-ring. A back-up ring is a ring that supports the O‑ring and is installed on the side receiving the least amount of pressure. If the O‑ring receives pressure from both directions, back-up rings must be installed on both sides of the O-ring. See Figure 7-13.

15. When pressure is applied to a Quad-ring®, it creates a dynamic seal by pressing against one side of the groove.
Most Quad-rings® are installed to fit tightly and securely into the groove between the inside of the cylinder barrel and the piston. When pressure is applied to a Quad‑ring®, it creates a dynamic seal by pressing against one side of the groove. See Figure This pressure forces the cylinder seal outward and against the sealing surface of the groove. The more pressure on a Quad‑ring®, the greater the sealing force.

16. Common cup seals include V‑ring seals and U‑ring seals.
Cup seals are available in various shapes to withstand different ranges of pressure. Cup seals are commonly used for sealing rotating or reciprocating shafts, pistons, piston rods, and pump shafts. Common cup seals include V‑ring seals and U‑ring seals. See Figure 7-15.

17. Compression seals are static cylinder seals commonly referred to as gaskets or packing.
Compression seals are static cylinder seals commonly referred to as gaskets or packing. See Figure A gasket is a seal used between machined parts or around the circumference of flange pipe joints to prevent hydraulic fluid leaks. A gasketed joint is sealed by compressing the gasket material into the imperfections of the mating surfaces of the joint. Gasket material can be plasticized rubber, synthetic leather, or soft metal such as brass or copper. Gasket material must be soft enough to form a proper seal when pressed between two harder materials.

18. A rod wiper prevents foreign materials on the rod from contaminating the hydraulic fluid inside a cylinder.
A rod wiper is the part of a cylinder that keeps foreign materials that have attached themselves to the rod from entering the cylinder and contaminating the hydraulic fluid. See Figure A rod wiper has a lip that wipes foreign materials from a rod with each stroke. Normally, rod wipers are installed under a threshold collar on the outermost portion of the rod end of a hydraulic cylinder. Rod wipers also protect the seal material in the rod-end end cap. Rod wipers are composed of synthetic material and are not designed to seal against pressure.

19. Cylinder mounting methods are categorized as fixed or pivot.
Cylinder mounting methods are categorized as fixed or pivot. See Figure A fixed cylinder mount is a mounting method that holds a hydraulic cylinder rigidly in place, only allowing rod movement.

20. A hydraulic motor converts hydraulic energy into rotating mechanical energy.
A hydraulic motor is a device that converts hydraulic energy into rotating mechanical energy. See Figure Hydraulic motors can change direction and vary speed quickly and are used to produce large amounts of torque capable of rotating heavy loads. For example, they are used in equipment such as concrete mixing trucks, wood chippers, and industrial shredders.

21. The farther a load is from the center of a hydraulic motor shaft, the greater the torque required to move the load.
Torque is a turning or twisting force that causes a shaft, or other object, to rotate. The presence of torque indicates that there is a force present, even without rotation. Torque is measured from the center of the hydraulic motor shaft to the point of force, which is usually the radius of a pulley. This means that the greater the torque, the farther the force is from the center. See Figure 7-20.

22. Changing displacement can allow for greater breakaway or starting torque, but it has an adverse effect on hydraulic system speed and operating pressure.
Changing displacement can allow for greater breakaway or starting torque, but it has an adverse effect on motor speed and operating pressure. See Figure Increasing displacement decreases motor speed and increases available torque. Decreasing displacement increases motor speed and decreases available torque. The available torque of a hydraulic motor is determined by the amount of pressure acting on the area of its rotating parts. The speed of a hydraulic motor is determined by the rate at which hydraulic fluid flows through the motor.

23. A gear motor has meshing gears, which are rotated by hydraulic fluid flow.
A gear motor is a hydraulic motor that has meshing gears, which are rotated by hydraulic fluid flow. See Figure The three different types of gear motors used in hydraulic systems are external gear, internal gear, and gerotor motors. A gear motor operates through the following procedure:
1. Hydraulic fluid flows into the inlet port of the motor at high pressure.
2. Pressurized fluid flow is converted into torque. As the gears in a gear motor rotate, a gear that is keyed with the drive shaft produces torque on it.
3. Hydraulic fluid is discharged through the outlet port at low pressures and back to the reservoir.

24. When hydraulic fluid enters the inlet of a vane motor, the resulting force causes rotation of the rotor and torque on the drive shaft.
A vane motor is a hydraulic motor that contains a rotor with vanes, which are rotated by fluid flow. When hydraulic fluid enters the inlet of a vane motor, the resulting force causes rotation of the rotor and torque on the drive shaft. See Figure A vane motor operates through the following procedure:
1. Pressurized hydraulic fluid flows through the inlet port and pushes against the vanes fitted into the rotor, causing rotation of the motor.
2. Rotation of the rotor produces torque on the drive shaft.

25. A piston motor has internal pistons, which are extended by hydraulic fluid pressure to produce rotational movement.
A piston motor is a hydraulic motor that has internal pistons, which are extended by fluid flow to produce rotational movement. See Figure Piston motors are the most efficient of all hydraulic motors. Piston motors are available as fixed or variable displacement and in bent axial form. Piston motors produce torque through fluid pressure acting on the pistons that are reciprocating inside a piston block.

26. Oscillators are used to provide instant torque at a high rate for moving an object a short distance, depending on the setting of the stop.
A stop is a stationary barrier used to prevent the continuous rotation of an oscillator. Depending on the setting of the stop, oscillators can provide instant torque at a high rate for moving an object a short distance. Oscillators are used in applications that require clamping, opening and closing, loading, transferring, unloading, or turning. See Figure Although there are several types of oscillators used in industrial applications, the most common type is the vane oscillator.

27. Rack and pinion valve actuators can be low-torque or high-torque designs.
Rack and pinion valve actuators can be low torque (for light loads) or high-torque (for heavy loads). See Figure Rack and pinion valve actuators are typically used in heavy-duty applications because they can handle heavy side and end loads and can accommodate large bearings. They are also used for precision control operations because they provide constant torque output and resistance to drift.