Unit Eight Check Valves, Cylinders, and Motors

1. Unit Eight Check Valves, Cylinders, and Motors Check valves can be compared to diodes in that they allow “flow” in only one direction. Check valves share a “parallel” relationship with components on order to allow a bypass path. Check valves fall into the family of directional control valves. In pneumatic and hydraulic systems the check valve works the same.

2. Check Valve Operation In the illustration above, fluid would move freely from the right to the left. Fluid is blocked from left to right.

3. Pilot Operated Check Valve The operation of this check valve is just like before except that it has the advantage of allowing flow in both directions. The dashed line in the illustration, which is the pilot line, is used to lift and “hold” the internal member off of the seat. This is a desirable feature in some machine applications.

4. Pilot Operated Check Operation This simple illustration shows that a piston and rod is used to hold the internal member off the seat. When fluid is released from the pilot port, the spring forces the piston and rod out of the way and the “poppet” closes.

5. How a P.O. Check Valve Operates in a Circuit The above illustration is for a “load lock” circuit. The check valve ensures that the load positively can not move until the operator wants it to.

6. Cylinders The schematic symbol, as well as the function, for a cylinder is exactly the same for pneumatics as it is for hydraulics.

7. Cylinder Construction A cylinder is not a very complicated thing. The basic construction is the same for pneumatic and hydraulic cylinders except that pneumatic systems tend to use cylinders with aluminum bodies and hydraulic systems use cylinders made with steel bodies, the reason being one of pressure.

8. Seals Seals serve to prevent the unwanted escape of pressurized fluids between the surfaces of mating components.

9. Cylinder Cushions Cylinders sometimes impact hard at the end of a stroke. To help alleviate this potentially damaging shock, devices called cushions are used inside the cylinder. Both pneumatic and hydraulic cylinders use cushions.

10. Cylinder Stroke Adjustment Although not used as much today, strike adjustors allowed for changing the effective stroke length of a cylinder in applications that required it.

11. Cylinder Mounting Styles Cylinder mounting refers to the way the cylinder is attached to the machine and has a direct bearing on its service life.

12. Mechanical Motion Normally cylinders produce a linear motion but, with internal or external modifications, cylinders can produce rotary motions as well.

13. Cylinder Loads A cylinder rod load is classified as “thrust” if pushing or “tension” if pulling. The type of load has a direct bearing on how the cylinders can be used in operation.

14. Stop Tube Stop tubes, also known as stop collars, are used on long stroke cylinders to protect the rod gland bushing fro excessive side load which in turn could cause the rod to bend. If for any reason the collar should need to be changed or repaired, the cylinder must be disassembled.

15. Common types of Cylinders Single acting cylinders are used where forced motion is only needed in one direction such as an in-ground car lift. Rams are used where excess force is needed and where side loading might be a problem. Double acting cylinders are used in most applications especially when forced motion is required in both directions. Double rod cylinders produce the same extending and retracting forces and speeds.

16. Common types of Cylinders Telescoping cylinders are used where they must occupy short linear distances when not in use and reach great distances when needed. Tandem cylinders give the advantage of increased force in a slender package. Duplex cylinders fit into tight places and give three possible positions since one of the cylinders has a rod shorter than the other.

17. Hydraulic Motors Hydraulic motors create a rotary motion and force. The amount of rotary force produced is directly related to the pressure applied and the size of the motor.

18. Torque Torque is often defined as a twisting turning effort found at a rotating motor shaft which is overcoming a load. The torque of a fluid motor can be calculated using the following formula: Motor Torque = Pressure x Displacement 6.28

19. Pound Inch In the above illustration, the amount of torque generated at the motor shaft is determined by the distance of the lever from the center of the shaft. The grater the distance, the grater the force. This is why large displacement motors generate more torque than smaller ones. Since torque is based on two units, pound for force and inch for distance, we use the pound inch to describe it.

20. Motor Drains Drains are used on hydraulic motors that turn in both directions because of internal leakage. This leakage, if allowed to build up and pressurize, could cause the shaft seal to blow out or leak. In the symbol it’s the addition of a dashed line that is returning to tank.

21. Vane Motors Vane motor operation is accomplished when pressurized fluid acting on the vanes pushes them out of the way to get to the outlet port which is at low pressure. In the process, the movement of fluid causes rotary motion at the shaft.

22. Balanced Vane Motor As in the case of the vane pump, the vane motor has an “elliptical” cam ring which allows four pressure quadrants to exist around the shaft bearing thus reducing side loading and increasing service life.

23. Extending Motor Vanes Extending vanes against the cam ring is necessary in order to form a positive seal. This is done either by mechanical means or by fluid pressure.

24. Fluid Extension of Vanes In the illustration above, fluid is forced under the vane before the inlet port to ensure loading of the vanes. A small check valve uses a calibrated spring, usually around 65psi.

25. Gear Motors Gear Motor Gerotor Motor External Gear Design Internal Gear Design Motors are classified as internal or external referring to the arrangement of their rotating group. Most external gear motors use the “spur” or “straight” tooth profile.

26. Piston Motor Operation In the picture above, fluid comes in and pushes the piston up the swash plate to create motion. As in the case of the pump, the swash plate determines displacement which affects speed and torque.

27. Overcenter Axial Piston Motor The above schematic symbol indicate a hydraulic motor that can change its direction of rotation by a internal mechanical adjustment. Where this might be accomplished by the use of a directional control valve, here it is done at the motor. Such motors are often used in hydrostatic drives.

28. Pneumatic Motors Above is the schematic symbol for a bi-directional pneumatic motor. Bi-directional means that the motor can rotate in two directions. The two arrows pointing inward determine it to be bi-directional and that they are transparent determine the component to be pneumatic.

29. Pneumatic Vane Motor Vane motors, the most commonly used motor for industrial tools, work on the same principle as do the hydraulic motors. To extend the vanes, pressurized air is directed under the vanes and they move out toward the cam ring to form a positive seal. These motors generally run at high speeds.

30. Additional Terms Starting Torque: The amount of torque required to overcome a load from the resting position. Breakaway Torque: The amount of torque required to overcome the motor’s own friction from the rest position. Both of these values are important for calculating a motor’s NET torque.