General Engineering Notes

Selection of the proper sized actuator for an application is accomplished by determining the necessary torque to move the load at the required speed, the available fluid pressure and the necessary arc of rotation. Good design practice dictates a nominal over - capacity be designed into the load moving system.

Load torque TL inch pounds) is the resistance to the movement of the shaft due to a load force or mass, M, (pounds) acting at a distance, R, (inches) from the center of the shaft rotation TL = MR.



Motion will occur when the applied torque of the actuator exceeds the load torque. The velocity and acceleration, A, given to the load mass, M, is proportional to the excess torque or force, F.



Similarly, the load mass once set in motion must be stopped or decelerated with an opposing force F=MA. This deceleration force can be obtained by gradually restricting the flow of fluid to and from the actuator.



Caution:
Actuator should be protected from over pressurization during deceleration.

Lifting a mass in an arc causes the effective radius ER, to vary with the rotational position, becoming minimum at the vertical (90º) position. The load torque due to load force thus decreases from maximum at position 1 to minimum at position 2, and then reverses to aid rotation from position 2 to position 3. Restrictions of fluid flow and control of deceleration pressures is vitally necessary in this type of application.

Calculation of the amount and rate of energy dissipation required to stop a moving mass is possible if the variables such as velocity, mass, time, pressure, viscosity, etc., can be determined. In actual circuits these factors are inter-related and solution is often complex.

Good general practice requires that more cycle time be allowed for deceleration than for acceleration of a given mass.

A simplified calculation can be made if the assumption is made that the acceleration and deceleration are constant and uniform. The energy required to accelerate the mass must be equal to the energy to decelerate the mass. This simplifies to the following formulas:

Example:
A mass accelerated uniformly for 50º @ 800 psi moves at constant velocity through use of flow-control valves until decelerated in the last 100º in 10 seconds @ 400 psi.
Note, however, that if the driving pressure were not removed during the deceleration period, the total deceleration pressure would be the sum of pressures, and at 1,200 psi could exceed the rating of the unit.

Actuator distributors can provide valuable assistance in solving specific circuit and application problems.

Direction and speed control for slow speed and light loading applications can be accomplished with relatively simple fluid circuits using hand- operated 4-way valves.



High speed and/or rapid cycling operation would suggest a commercially available solenoid-operated 4-way directional control valve and flow-control valves for better control of cycle motions, and the addition of fluid cooler, accumulators, and other components directed to specific system requirements.



Severe shock and possible damage to the system can occur on hydraulic applications by sudden or complete restriction of outgoing fluid, which allows the moving mass to generate high surge or transient shock wave pressures which must not exceed the rating of the unit.

Deceleration valves, actuated by cams or by limit switches, are often used to gradually restrict the fluid and stop the moving mass. Usually, relief valves plumbed as shown, or plumbed from one line to the other in each direction, will limit the generation of surge pressures to a safe value. Cross-port relief manifolds are available for most actuators. If cam valves are used, the cam shape should provide a gentle ramp transition, and the spool should be tapered to provide a gradual closing off of fluid. As a general rule, external stops, mounted securely to the machine framework, should be used to stop the load. The shaft vanes should not contact the internal stops except under very light loads.

Air bleeding in hydraulic systems is usually not required if actuator is mounted with supply ports upward. In other positions, air will gradually dissolve in the oil and be carried away as the actuator is cycled. Special bleed connections are available as an optional feature on some actuators if specified when ordering.

Internal by-pass flow is always present to a small degree, and increases with increase of pressure. On air applications it must be recognized that on stall-out applications, under air pressure, there will be a small continuous by-pass flow.
Pure torque out-put from the actuator without external radial shaft bending loads is preferred to allow maximum bearing life. An arrangement with a semi-flexible coupling and the load shaft supported by separate bearings is recommended.



A similar arrangement is advised for power transmission through gears to eliminate gear load and separating forces from aggravating the actuator bearing load.



Where a flexible coupling cannot be used, very accurate alignment of the actuator and associated equipment is essential to prevent undue actuator bearing loading.
End thrust or axial loading of the actuator shaft is not advised. A thrust bearing, and the load driven through a sliding spline (or other means) is recommended to minimize internal wear for maximum actuator life.



Temperature:
Standard actuators, unless otherwise specified, may be operated satisfactorily between minus 30ºF and plus 250ºF. Operation at higher temperatures requires special seal compounds.

Filtration:
Filtration of operating fluid to the 25 micron range is recommended.

Storage:
Actuators, when stored for any extended period of time, will require additional rust protection. Upon receipt of the actuator, remove port plugs, fill the actuator chambers with clean, mineral-base oil (or other fluid compatible with seal compounds), and replace plugs securely. Cover exterior surfaces with adequate rust-preventive material. Place in a poly bag and seal.

Installation:
Normal machinists' practice and care should be used in installing actuators. As for any oscillating type actuator, the most efficient means of transmitting the torque developed is through multiple tooth, involute spline or SAE 10-B spline. Suitable flange type adapters and straight connectors are covered under "Accessories" in the catalog. These are also available through the local distributor.

System Pressure:
Caution must be exercised in actuator sizing by making allowance for a pressure drop throughout the hydraulic system in which the actuator is installed. If an extensive system of piping, control valves, flow control valves, etc. is present, it is to be expected that full line pressure will not be available at the actuator inlet port.

Angular Velocity:
Angular velocity can be readily controlled by metering the amount of flow of fluid into or out of the actuator ports. Many designs of flow control valves are available on the market for this purpose. If greater flow is required than that available in the selected standard actuator, special larger size ports can be specified within reasonable limits.

Service and Repair:
Seals in actuators are readily replaced by qualified personnel trained in hydraulic equipment repair. Interchangeable replacement parts are available from factory. Always specify the serial number and bill of material of unit when ordering spare or replacement parts. Replacement of worn bearings may be accomplished by qualified personnel, but we recommend that such repairs be made by the Factory Repair Department so that units can be reconditioned to meet original performance specifications.

Distributors in principal cities throughout the U.S., Canada, Europe, and Asia can supply you with additional information. If you have any questions, contact your distributor, or the actuator factory.

An overhaul procedure which contains complete instructions for replacement of seals or other worn parts, and an exploded view and parts list for ordering replacement parts, is available from the factory.

Service operations should be performed by competent hydraulic equipment technicians to maintain high manufacturing quality standards.


L = Body Length (in.)
D = Body I.D. (in.)
d = Hub dia. (in.)
ARC = Degrees of Rotation
N = Number of Vanes
PSI = Lbs/Sq. Inch (Pressure)
Displacement Per Radian = [N•L(D2 - d2)]÷8(in3/Rad.)
Theoretical Torque = [N•L(D2 - d2)÷8]PSI (in.-lb.)
Actual Torque = Theoretical Torque • % efficiency (in.-lb.)
Total Displacement = [LARCNπ(D2 - d2)]1440(in3)