Getting your system in Sync Part 2
A rephasing hydraulic cylinder is essentially a standard cylinder with the addition of one important feature — built-in orifices or passageways that hydraulically resynchronize cylinder motion, should they come out of synchronization. These orifices can be on the cap end of the cylinder, the rod end, or both. There are several methods to accomplish this function. However it is accomplished, this orifice permits a small amount of hydraulic fluid to bypass the cylinder piston in the fully retracted or fully extended position. This make-up fluid allows the cylinders to rephase (synchronize) themselves, if necessary, during operation, Figure 1.
Rephasing cylinders are typically used in one of two operating modes: series or parallel.
Series mode — In this mode, the bore and rod diameters of the cylinders are of a size such that all cylinders extend or retract an equal distance when fluid flows into the first or last cylinder in the series. This type of circuit has a wide range of functions on many different types of machines.
Parallel mode — Here, as shown in Figure 2, the bores and rod diameters of the two cylinders are the same. Therefore, Cylinder A will extend the same amount as Cylinder B retracts when fluid enters the cap end of Cylinder A. Similarly, Cylinder A will retract the same amount as the right cylinder extends when fluid enters the cap end of Cylinder B. This type of circuit is typically used for the steering function on a mobile machine.
Advantages of the rephasing approach
There are several advantages to using rephasing cylinders. They offer very high dividing/ combining accuracy, typically two to four times that of valve-type divider/combiners, and equal to or better than external gear rotary-type divider/combiners. Because they combine the functions of the dividing and actuator element into one component, cost is reduced. Other benefits include:
Elimination of the need for a mechanical connection between the actuators — the dividing/combining accuracy of rephasing type cylinders is typically high enough that it eliminates the need for any type of external mechanical connection or torque tube in the application. The machine operator is easily able to resynchronize at any time during operation by simply fully extending or retracting the cylinders.
Pressure intensification — Because a rephasing cylinder is a positive-displacement type device, effective power transfer between actuators is possible due to the pressure intensification effect, just as with the external gear rotary flow dividers. This is true for both series and parallel operation.
No external connection, device, or plumbing to achieve hydraulic resynchronization — The resynchronization is done internally within the cylinders., helping minimize cost and size.
No constant parasitic power loss — Resynchronizing orifices are placed at the fully extended or fully retracted positions. Thus, the potential for parasitic power loss exists only at either or both of those positions, and not at any other stroke positions.
Disadvantages of rephasing cylinders
There are a few drawbacks to rephasing cylinders. They can only be used in linear actuator applications. There is a potential need for protection from the pressure intensification effect. Depending on design and system parameters, pressure intensification may cause pressures to exceed the working pressure ratings of the cylinders themselves, or other components within the system. This may require additional components (i.e., relief valves, plumbing, etc.) for pressure protection, adding cost to the system.
There is also slightly less design flexibility. A rephasing cylinder combines the functions of actuator and divider/combiner into one component. Thus, machine designers and manufacturers have no ability to (physically) place the divider/combiner in a location separate from that of the actuator itself, should that be desirable.
Recent improvements
Rephasing type hydraulic cylinder technology has existed since the late 1960s or earlier. However, recent design innovations have made it more dependable and reliable. Following are some of the basic design methods that have been used.
The bore of a high-quality hydraulic cylinder may have a surface finish as fine as 0.1 to 0.25 µm. Such a smooth finish is required to achieve effective piston sealing at pressures to 10,000 psig in a standard cylinder. Any time the soft piston seals travel across a hole intersecting the tube, the possibility of piston seal damage exists, possible causing cross-piston leakage.
Slotted wall design — this is another method of achieving a rephasing orifice, by milling a small slot into the inner wall of the cylinder tube, parallel to the axis of the tube, Figure 4. This improves reliability, as the slot is milled into the tube with a special tool in parallel with the axis of the tube (as opposed to drilling a perpendicular hole). The radius of the slot creates a natural ramp to ease the piston seal across the slot.
The slotted wall design provides less chance of damage to the piston seals when they pass across the slot during extension or retraction. However, milling the slot is a slightly more expensive operation than drilling a small hole. And even with the slotted design, the piston seals still pass over a surface with a smoothness or finish inferior to that of the inside wall of the cylinder tube itself. This still can cause damage to the piston seals, although the likeli-hood is much less than that for the drilled hole design.
Rephasing valve within cylinder piston — with this design, a small, double-acting valve is used instead of a static orifice, Figure 5. Here, the piston seals do not need to pass over a hole or slot to achieve the rephasing function. Consequently, there is no chance of piston seal damage. The result is greater reliability and longer life. The rephasing valve design shown has been successfully endurance tested in excess of 100,000 extend/ retract cycles. There is also a higher working pressure capability. The piston seal of an orifice or slot-type rephasing cylinder will typically extrude or erode if exposed to pressures over 2500 psig. The rephasing valve design cylinders have no such pressure limitation. Another benefit is that use of better piston seals becomes possible: urethane piston seals can be used instead of PTFE. Urethane offers superior wear resistance in such applications. Traditionally, PTFE piston seals have been used in rephasing cylinders with the drilled hole or slotted wall designs because they cross over holes or slots in the tube with less propensity for damage (as opposed to nitrile, polyester, or urethane). The low friction offered by the PTFE seals is the characteristic that makes this possible.
Lastly, there is a faster rephasing response. If an orifice or slot type rephasing cylinder is in its rephasing position and must move a load, there is a brief period where fluid will inadvertently bypass the piston before the seal closes off the rephasing hole or slot. The rephasing valve has a quicker response, and as such minimizes inadvertent bypass flow. However, as might be expected, the rephasing valve is the most expensive option of the three main designs discussed here.
Contact author Ken Rosenbecher at [email protected] or visit www.energymfg.com.
