Terms and Definitions Cylinder Actuators Symbols for Actuators Terms and Definitions II Cylinders Providing Linear Motion Cylinders Providing Angular Motion Parts of Actuators Mounting of Actuators Seals Used on Cylinders and Actuators Sources of Common Failures of Hydraulic Cylinders Terms and Definitions III Pascal s Law Calculating Force Capability in a Single- Acting Cylinder Terms and Definitions Cylinder is an actuator that converts hydraulic power into linear mechanical force. Linear means extending or moving in one dimension only. Load is the resistance the hydraulic system must overcome in order to function as designed. Pinion is a gear or cog with a number of small teeth and is used with a rack to convert linear to circular motion. Ports are openings. Calculating Force Capability in a Double- Acting Cylinder Calculating Cylinder Speed Types of Hydraulic Motors Hydraulic Motor Symbols Calculating Torque Formula for Calculating the Displacement of a Hydraulic Motor Formula for Calculating the Speed of a Hydraulic Motor Formula for Calculating Hydraulic Motor Pressure Formula for Calculating Hydraulic Motor Power Formula for the Calculation of Motor Overall Efficiency Measuring Flow Rate Rack is a bar with teeth and is used with a pinion to convert linear to circular motion. Rotary means turning, or capable of turning, on an axis. Sequentially means in order. Telescoping is extending from nested sections. Torque is rotational force. Vane is a blade, attached radially to a cylinder, that moves (or is moved by) hydraulic fluid or air. Cylinder Actuators Single-acting cylinder. Oil flows into one side of the cylinder, normally to extend cylinder. Can be retracted by oil flow instead. Cylinders returned to original positions by either load or spring. Telescoping cylinder. Single-acting cylinders. Have two or more sections that extend sequentially. Double-acting cylinder. Oil flows into one port to extend cylinder. Oil flows into other port to retract cylinder. Example is the packer-ejector in a trash truck. 2012 Jones & Bartlett Learning 1
Vane type of rotary actuator is similar to a hydraulic motor, except that the rotation is limited to 300 or less. How it works. Normally two oil ports. Oil flow rotates actuator clockwise or counterclockwise. Depends on direction of flow. Provides torque as force output to load. Rack and pinion rotary actuator utilizes a linear piston (rack) with gear teeth on its shaft. Rack is driven by oil entering one of the ports at each end of the piston. Pinion is a gear attached to the output shaft that is rotated by the movement of the rack. Shaft can be rotated from a few degrees to more than 360, depending on the stroke of the piston and the number of gear teeth. Torque. The action provides torque as its output force to the load. To increase output torque capability, two racks may be used to rotate pinion. Hydraulic motor. Similar in design to hydraulic pump. May include same components as hydraulic pump. How it works. Provides continuous rotation (in excess of 360 ) to maintain constant flow. Uses fluid power input to provide mechanical power output. By contrast, a hydraulic pump uses a mechanical power input to produce a fluid power output. Symbols for Actuators Learn how to identify common types of actuators by their symbols. Many variations on these basic symbols exist. Consult any hydraulics textbook or perform a simple keyword search online for additional examples. Terms and Definitions II Angular means consisting of, or forming, an angle. Trunnion is paired cylindrical projections used for support, as on a cannon. Bore is the inside diameter of a tube. Housing is an enclosed case for a mechanism. Mount is a support on which a piece of machinery is attached. Piston is the solid disk that moves within a tube (or cylinder) under fluid pressure. Rod is a slender bar, or pole, often made of metal. Seal is something used to completely close a gap, seam, or opening. Teeth are the uniform projections in a piece of machinery that engage and transfer motion to or from a complementary piece of machinery. Vent is an opening used to release or discharge a fluid or gas. Clevis is an eye in a hydraulic mount, which is secured with a bolt. Flange is a ring or collar used to increase strength and provide a place to attach other objects. Composite means to be comprised of several substances. Elastomeric is a natural or synthetic material that returns to its original shape after a deforming force is removed. Metallic means to be comprised of metal. O-ring is a flat gasket used to seal against high pressure. Scored means to be notched, scratched, or incised. 2012 Jones & Bartlett Learning 2
Cylinders Providing Linear Motion Cylinders mounted rigidly. Cannot move from original alignment. All motion results from extension or retraction. Motion is linear and along center line of cylinder. Cylinders Providing Angular Motion Trunnion mounted. Cylinder free to move from original alignment. Allows load to move in arc. Example is a cylinder that extends the bed of dump truck. Provide limited rotation to load. Example is a device in production line. Picks up a part. Rotates to deliver it to machining operation. Rotates back to pick up next part. Parts of Actuators Single-action cylinders. Cylinder housing or barrel. Cylinder bore. Rod. Oil port. Piston. Piston seal. Rod seal. Air vent. Rod wiper seal. Mount. Double-acting cylinders. Cylinder housing or barrel. Cylinder bore or ID. End caps. Oil ports. Piston. Piston seals. Rod. Rod pressure seal. Rod wiper seal. Mounts. Vane-type rotary actuators. Housing. Oil ports. Vane. Vane stops. Output shaft. Rack-and-pinion rotary actuators. Rack. Rack piston. Rack teeth. Pinion. Pinion gear. Output shaft. Hydraulic motor. Driving gear. Gear teeth. Inlet port. Outlet port. Housing. Driven gear. Chamber. Mounting of Actuators Mechanisms that produce linear motion typically use a four-bolt mount, either foot-mounted or with flanges on either the blind end or the rod end. Mechanisms that produce angular motion are typically mounted using clevises through which bolts attach them to the machine. 2012 Jones & Bartlett Learning 3
Seals Used on Cylinders and Actuators O-rings. Flat gasket. For pressures up to 1,500 psi. High friction. Piston rings. Similar to engine piston rings. Produce least friction. Prone to leakage. Cup seals. High pressure. Very tolerant of out-of-round cylinder barrels. Seal in only one direction, so two seals needed. Lip seals. V-ring (chevron) or U-ring. Low pressure individually. Can be stacked to seal up to 10,000 psi. T-seals require backup rings to seal. Seal materials used are elastomeric, metallic, and composite. Sources of Common Failures of Hydraulic Cylinders Leaking rod seal. Defective rod wiper seal. Scored cylinder barrel. Nicked or damaged rod. Bent rod. Note: Defective wiper seal may cause leaking piston seal, leaking rod seal, or scored cylinder barrel, because it allows contamination to enter the cylinder. Terms and Definitions III Effective area is the net surface area used in a force or pressure calculation. Pascal s law is the mathematical relationship between force, pressure, and area. Pi (p) is the ratio of the circumference of a circle to its diameter. Displacement is the amount of fluid that is transported from the motor inlet to the outlet during one revolution of the motor. Efficiency is the ratio of work done to energy required, expressed as a percentage. Pascal s Law Pressure applied anywhere to a body of fluid causes a force to be transmitted equally in all directions. Mathematical expression is force equals pressure multiplied by area or F = p A. Invert the formula to solve for pressure (pressure equals force divided by area) or p = F/A. Force is measured in pounds (lb), area is measured in square inches (in 2 ), consequently, pressure is measured in pounds per square inch (psi). Calculating Force Capability in a Single-Acting Cylinder In order to solve force capability problems involving cylinders, one must first determine the area of the piston head. Calculate the area of the cylinder (area equals Pi multiplied by the square of the cylinder radius) or A = pr 2. Note: Always approximate π consistently throughout an equation or series of equations. Note: The calculation of radius when you know the diameter of the cylinder (the bore) is radius equals half the diameter of the cylinder or r = d/2. Use the cylinder s area to solve for force (or pressure) in Pascal s equation. The symbol means approximately, which can mean the number has been rounded. 2012 Jones & Bartlett Learning 4
Calculating Force Capability in a Double-Acting Cylinder Force capability is different on extension and retraction. On retraction, the area subjected to hydraulic pressure is lower than on extension, because the area taken up by the rod is not used to move the load. On extension, the effective area is the entire face of the piston: Apiston = πr 2. On retraction, the effective area is the piston area minus the rod area: Aeffective = Apiston - Arod. Rod area is calculated like piston area: Arod = πr 2. Note: Use subscripts in your formula to prevent confusion. Retraction force: F = p Aeffective. Calculating Cylinder Speed Speed at which cylinder extends or retracts is a function of the flow rate of oil entering or leaving the cylinder. For now, consider flow rate entering the cylinder. General equation for speed (velocity) of cylinder is v = 3.85 Q/A, where Q is the flow rate in gallons per minute (gpm), and A is the area available in square inches. The answer is in inches per second (in/sec). Note: When using this equation with the 3.85 factor, the flow rate must be in gpm, and the area must be in square inches. A double-acting cylinder has more force capability on extension than on retraction for any given pressure. A double-acting cylinder has a faster retraction speed than extension speed for any given flow rate. The force produced by a cylinder is purely a function of pressure. Flow rate is not considered. The speed of operation of a cylinder is purely a function of flow rate. Pressure is not considered (assuming that there is sufficient pressure to move the load in the first place). Types of Hydraulic Motors Vane hydraulic motor. Gear hydraulic motor. Piston hydraulic motor. Hydraulic Motor Symbols Fixed displacement. Bidirectional. Variable displacement. Calculating Torque Since rotary actuators do not produce linear forces as cylinders do, we need to look at the output force from these devices. These actuators produce torque as their output force. Torque, or rotary force, is calculated by the equation torque equals force times distance, or T = F d. Measurements of torque are expressed in poundinches (lb-in). To convert lb-in to ft-lb, multiply the answer by 12. Formula for Calculating the Displacement of a Hydraulic Motor Displacement is the amount of fluid that is transported from the motor inlet to the outlet during one revolution of the motor. D = 231 Q/N, where D is displacement in units of cubic inches per revolution (in 3 /rev), Q is motor inlet flow rate in units of gpm, and N is shaft speed in units of revolutions per minute (rpm). 2012 Jones & Bartlett Learning 5
Formula for Calculating the Speed of a Hydraulic Motor N = Q 231/D, where N is shaft speed in units of rpm, Q is motor inlet flow rate in units of gpm, and D is displacement in units of in 3 /rev. Formula for Calculating Hydraulic Motor Pressure p = T 2π/D, where p is the inlet pressure in units of psi, T is torque (shaft torque) in units of in-lb, and D is displacement in in 3 /rev. Formula for Calculating Hydraulic Motor Power To calculate hydraulic motor power, first calculate input power. P = p Q/1714, where P is input in units of horsepower (HP), p is the inlet pressure in units of psi, and Q is inlet flow in units of gpm. Then calculate output power. P = T N/5252, where P is output power in units of HP, N is shaft speed in rpm, and T is output torque in ft-lb. Formula for the Calculation of Motor Overall Efficiency Efficiency is the ratio of work done to energy required, expressed as a percentage. Overall efficiency is output power divided by input power, or Eff = Output power/input power. Measuring Flow Rate Orifice flow meter determines the flow rate by measuring the pressure drop across an orifice of known characteristics. Turbine flow meter, flow through the meter causes the turbine to spin, and a sensor detects the turbine speed that is then used by a built-in computer to calculate and display the flow rate. Variable area flow meter, flow through the sharpedged orifices in a magnet/piston assembly results in a differential pressure, generating force. Force causes assembly to push against calibrated spring. Distance assembly moves are translated into flow rate. Shown by flow indicator on outside of body. 2012 Jones & Bartlett Learning 6