Sizing of hydraulic tubing
October 2002
Pneumatics & Hydraulics
When determining tubing sizes one must always use the flow rate that is going to be transmitted in the tubing for calculating diameters. Do not use the pump inlet and outlet ports as a guide to determine the tube size. One can safely say then, that any component used in a hydraulic system must be chosen according to the flow it can handle and not according to port sizes. There are recommended flow velocities for the various lines in a hydraulic system. These fluid velocities are recommended for oil up to a viscosity of 68 centistoke at 40°C. The maximum fluid velocity for pump intake is 0,7 m/s (suction line). Fluid velocity for return lines is 3 m/s. The flow rate of oil leaving the cap end of a cylinder, when it is retracting, is more than the oil that is being delivered by the pump. The reason for this statement is that the oil is being forced into the rod-end of the cylinder, against the annulus area which is smaller than the full piston area. This means that the cylinder retracts at a faster rate than it extends, sending a higher flow rate out of the cap-end of the cylinder, which must be taken into consideration when determining the size of the return line.
Formula to determine the inside diameter of hydraulic tubing:
Where
D = inside diameter of tubing in millimetres
Q = oil flow rate in litres per minute
V = recommended fluid velocity in metres/s.
Useful hydraulic formulae
Pumps and motors:
Power to drive hydraulic pump (kW) = pressure (MPa) x flow (l/s) x 1,2 (extra power to overcome frictional losses).
Geometric flow rate of pump/motor (lpm) = Swept volume (cm3/rev) x shaft speed (rpm)/1000
Theoretical torque of hydraulic motor (Nm) = pressure (N/m2) x swept volume (m3/rev)
Heat equivalent (Joule/s) = pressure (N/m2) x flow rate (m3/s)
Cylinders
Flow rate (m3/s) = effective piston area (m2) x cylinder speed (m/s)
Theoretical force (N) = pressure (N/m2) x effective piston area (m2)
For more information contact Hydraulic Steel Tube, 011 394 2966, allan@hst.co.za, www.hst.co.za
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