engineering and design tips for hydraulic cylinders

Hydraulic Cylinders

Engineering and Design Tips

rstom03@aol.com

 

 

Hydraulic Cylinders convert hydraulic pressure and hydraulic oil flow into force and motion. We have included this page to assist our customers in selecting cylinders and determining their performance in the overall hydraulic system. We strongly recommend that our customers discuss all aspects of their hydraulic cylinder requirement including the overall hydraulic system parameters with our engineering department to ensure a successful integration of the actuator into the machine.     Above: A cut away picture of a typical welded body hydraulic cylinder labelled with correct technical names for various components. Click on the image to open a larger version in a new window. To calculate hydraulic cylinder force output: F=PA where: F= Force in Lbs P= Pressure in psi A= Piston Area in Square Inches Note: On retraction subtract the rod area from the total piston area to determine net effective piston area. This will always be a smaller net retraction force on a rod cylinder. The larger the rod, the smaller the force. Note: Certain hydraulic systems, such as regeneration circuits, affect the output force of a cylinder by limiting the effective area of the piston. The overall system design must be considered when determining cylinder performance. To calculate required system hydraulic pressure: P= F/A Where: F= Required Cylinder Force in Lbs to Accomplish Work P= Required System Pressure in psi A= Effective Piston Area in Square Inches Note: Always add at least 30% to this resultant required pressure to allow for system pressure losses, friction and calculation errors. To calculate required hydraulic cylinder piston area: A= F/P Where: F= Required Cylinder Force in Lbs to Accomplish Work P= Available System Pressure in psi A= Required Effective Piston Area in Square Inches   www.vphyd.com   To calculate piston areas: A=(π)R² or A=(π)D²/4 Where: A= Piston Area in Square Inches R= Piston Radius in Inches D= Piston Diameter in Inches π= Geometrical Constant Pi = 3.14159 To calculate rod cylinder effective retraction area: Ae= Ap - Ar Where: Ae= Effective Retracting Piston Area (Rod End) in Square Inches Ap= Total Piston Area (Cap End) in Square Inches Ar= Area of Piston Rod in Square Inches To calculate required cylinder bore size: D= √(4A/π) Where: D= Required Cylinder Piston Diameter (Bore Size) in inches A= Required Effective Piston Area in Square Inches Note: For determining the bore of a cylinder used in the pull or retraction stroke, this area will be the total sum of the rod area and the effective piston area. √= Square Root of the following result π= Geometrical Constant Pi = 3.14159 To calculate cylinder oil volume: Vc= A x S and Vr= Ar x S Where: Vc= Total Cylinder Volume in Cubic Inches in the Rear or Cap End A= Piston Area in Square Inches S= Cylinder Stroke in Inches Vr= Piston Rod Volume Ar= Area of Piston Rod To calculate cylinder rod end volume: Vf= Vc - Vr Where: Vf= Cylinder Volume on the Front End (Rod End) of the Cylinder Vc= Cap End Cylinder Volume Vr= Piston Rod Volume   To convert cylinder volume into gallons: Vg= V/231 Where: Vg= Volume in US gallons V= Cylinder Volume in Cubic Inches To calculate cylinder extension rod speed: Rse= Q x 231/Ae Where: Rse= Cylinder Rod Speed on Extension in inches/minute Q= Hydraulic Pump Flow going to the Cylinder in GPM Ae= Cap End Piston Area in Square Inches Rse= Q x 231/720 x Ae Where: Rse= Cylinder Rod Speed on Extension in feet/second Q= Hydraulic Pump Flow going to the Cylinder in GPM Ae= Cap End Piston Area in Square Inches To calculate cylinder retraction rod speed: Rsr= Q x 231/Ae Where: Rsr= Cylinder Rod Speed on Retraction in inches/minute Q= Hydraulic Pump Flow going to the Cylinder in GPM Ae= Effective Retracting Piston Area (Rod End) in Square Inches Rsr= Q x 231/720 x Ae Where: Rsr= Cylinder Rod Speed on Retraction in feet/second Q= Hydraulic Pump Flow going to the Cylinder in GPM Ae= Effective Retracting Piston Area (Rod End) in Square Inches Note: Because the rod end volume of a rod cylinder is smaller than its cap end volume, a rod cylinder will retract faster than it will extend because the pump flow will fill up the smaller rod end volume quicker. It will, of course, also have less force in retraction than extension due to the smaller effective piston area on the rod end side. This faster retraction speed will also cause a return flow from the cap end of the cylinder that is much larger than the pump flow. This will affect the selection size of the return line filter.   To calculate required hydraulic pump flow for known required cylinder speed: Q= Rs x Ae /231 Where: Q= Required Hydraulic Pump Flow going to the Cylinder in GPM Rs= Cylinder Rod Speed in inches/minute Ae= Effective Piston Area in Square Inches Note: If required rod speed is known in inches/second, then multiply it by 60 to determine rod speed in inches per minute. Note: If required rod speed is known in feet/minute, then multiply it by 12 to determine rod speed in inches per minute. To calculate required size of return lines and return line hydraulic filter: The hydraulic flow returning to the system reservoir is often much higher than the output flow of the hydraulic pump. Because a rod cylinder will retract much faster than it will extend, the result is a large flow from the cap end during retraction. The larger the piston rod, the larger the return flow from a cylinders cap end. This return flow will determine the required port size of a cylinder, the required line size going to a cylinder, and the required volumetric capacity of the systems return line filter. Qmax= Rsr x Aere / 231 Where: Qmax= Maximum Hydraulic Flow going to the Reservoir in GPM Rsr= Cylinder Rod Speed on Retraction in inches/minute Aere= Effective Rod End Piston Area in Square Inches (Total Piston Area minus Rod Area) To calculate required motor Horse Power to drive the hydraulic pump: HP= Qs x P / 1714 x Eff Where: HP= Required Electric Motor Continuous Rated Horse Power Qs= Required Pump Hydraulic Flow in GPM P= Required Pump Hydraulic Pressure in PSI Eff= Overall System Efficiency (expressed as a decimal) To calculate the hydraulic pump outlet flow: Qp= RPM x Dp / 231 Where: Qp= Pump Output Hydraulic Flow in GPM RPM= Pump Rotational Speed in Revolutions per Minute Dp= Pump Displacement in Cubic Inches per Revolution To calculate hydraulic fluid velocity through the piping: Vf= 0.32 x Q / Ap Where: Vf= Hydraulic Fluidic Velocity in Feet per Second Q= Hydraulic Fluid Flow Rate in GPM Ap= Internal Area of the Piping, Hose or Tubing in Square Inches Other Hydraulic Cylinder Design Considerations: There are a number of other considerations when selecting the appropriate hydraulic cylinder for a task. These include: Rod Diameter based on required column strength. Appropriate Mounting Style for the application. Cylinder side loading and bearing requirements. Materials of Construction. Seal Materials. Port Locations, size and type. Cylinder coatings. Type of hydraulic fluid to be used. Other features such as Stroke Sensing.   Telescopic Cylinders. Often a cylinder is required to fit into a tight space and provide a stroke that is actually longer than its fully retracted length. A standard rod cylinder can not do this. A telescoping cylinder, however, can achieve this. Telescopic cylinders use a series of nested tubular rod segments. This enables a long working stroke in a short retracted length. These cylinders are available in 2, 3, or even up to 6 stages.     770-316-4951