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Continuously Connected Systems


A continuously connected hydraulic workholding system is one in which clamping pressure is derived from a continuous connection to a hydraulic power source. Such a system can range from a rudimentary welding fixture with a dozen clamps that are activated and maintained in position by a small stand-alone hydraulic power unit, to a sophisticated system using utilizing controls and hydraulic power units commonly found on today’s machining centers.

0637_appl A stationary fixture with continuous supply.

A key benefit of this type of system is that constant clamping pressure is assured because of the continuous hydraulic supply—there is no loss of pressure. Frequent applications of these types of systems are those in which the workholding fixture remains in the machining center for long machining cycles. Another example is when fixtures are dedicated to a particular machining center and not often changed. These fixtures can be single-sided, stationary fixtures or multi-sided rotating fixtures.

Two_side_fict A two-sided rotating fixture. A vertical fixture like this is often referred to as a tombstone.”


Because hydraulic lines are continuously connected to the fixture, care must be taken to insure the lines do not interfere with the machining process. Typical connection methods can be as simple as one hydraulic hose connected directly to the fixture or multiple lines connecting through the top or bottom of the fixture to control multiple circuits on the fixture. If the fixture must be able to rotate, a rotary union is required.

Rotary Unions

four_passage_rotation.gif Four- and one-passage rotary unions.

A rotary union is a device used to transfer hydraulic fluid from a stationary device to a rotating device where typical hydraulic lines would become twisted. Enerpac offers one-, two-, and four-passage rotary unions. The internals of rotary unions are arranged in “layer cake” fashion. Oil passages enter from one end of a cylindrical central core and pass along the core interior to open into individual, circular galleries arranged like layers of a cake. The galleries are separated from each other by seals.

Rotary_unions Cross-section drawing of a typical four-passage rotary union.

Hydraulic Sources

There are two types of hydraulic sources used in continuously connected systems.

Machining Center Powered

One approach is to use the hydraulic power unit supplied on the machining center. This type of hydraulic power supply often operates at pressures below 1500 psi with flow rates above 1 gpm, whereas workholding systems commonly operate at lower flows with considerably higher pressures.

Therefore, flow and pressure have to be considered when utilizing machining center hydraulics. These power supplies are designed to provide hydraulic pressure and flow to a wide range of components on the machining center, but are not always an ideal match to the hydraulic workholding system. Maximum flow rate requirements of some workholding components may necessitate use of flow controls.

Onboard units may also be limited in valving options necessary to control workholding fixtures. Workholding circuits typically require proper sequencing of location, support and clamping cylinders to ensure correct workpiece finishes and tolerances. The circuit may also require multiple pressure settings for different components on the fixture. These valving requirements normally must be supplied in addition to the standard hydraulic power supply.

Hydraulic Intensifiers

Intensifier.jpg Cross-section drawing of a typical four-passage rotary union.

Most Enerpac workholding components are rated at 5000 psi, which allows the use of compact components that achieve high forces. In order to fully realize the benefits of these components it may be necessary to use hydraulic intensifiers to increase pressure. Hydraulic intensifiers use low pressure flow from the onboard hydraulic power supply and increase it to a pressure range more suitable for clamping components. Intensifiers are small, quiet, and can be installed in any circuit where a low pressure power supply is present.

In clamping applications, high pressure is required for only a small portion of a machine cycle. An intensifier starts to work only when there is a need for high pressure. The rest of the time it is passive, has no internal consumption, and thus does not influence the performance of the machine system. When oil is supplied to the intensifier, it starts to build up the required end pressure. When that pressure has been reached, the intensifier stops and operates only as needed to maintain the pressure—for example to compensate for consumption or a leak. (Internal bypass valving enables high output flow rates during the early stages of pressure buildup.) Tip: The high volume flow of a typical onboard system can increase the intensifier’s stroke rate to a level at which its life is reduced. In such systems, it is a good idea to place a flow control valve before the intensifier.

Dedicated Power Supply

The second approach is to use a dedicated hydraulic power supply to feed the workholding system. Stand-alone units are often rated at higher pressures and lower flows than machine tool hydraulic systems. As a result they are a better match to work with standard workholding components. Separate units allow more flexibility in controlling workholding fixtures. They can be configured with various valving options and controls necessary to control workholding fixtures.

Power_unit.jpg Cross-section drawing of a typical four-passage rotary union.

Stand-alone units can also be used to power workholding systems on several machines at the same time. From a centrally located power unit, lines are routed to individual machines. (Floor troughs are an ideal, out-of-the-way method.) Control valves located on each machine allow the flexibility of individual control.

System Considerations

When using a continuously connected hydraulic system you must consider how the power unit will maintain pressure. Two approaches are commonly used. On systems where the machining cycle is short, it is common to keep the power unit running continuously, periodically shifting a directional control valve as needed to maintain pressure. This prevents unnecessary wear on the motor starter. On systems with longer machining cycles the hydraulic power unit can be turned on and off as need to maintain pressure. In either case it is necessary to incorporate a pressure monitoring device, such as a pressure switch, with necessary outputs to control either the valve or the pump motor. In more complicated systems, additional pressure and proximity sensors can be incorporated to increase automation of the circuit. Enerpac offers an “auto on/off” feature on several workholding electric power units. For longer machining cycles, these pumps can be ordered with a porting block and pressure transducer. They are set to turn the motor on when a preset “low” pressure is reached, and off when a preset "high" pressure is reached. For shorter machining cycles, the power unit can be ordered with an electric valve and pressure transducer. In this configuration, the pump motor runs continuously and the valve shifts to build pressure when the “low” setting is reached. When a preset “high” pressure is reached, the valve turns off.

Please contact Enerpac for additional assistance in configuring a continuously connected system.