Hydraulic clamping technology

Hydraulic positioning, clamping and supporting.

Reduction of downtime is an important factor in increasing productivity and maximising capacity utilization. Positional accuracy and the repeatability of clamping force also make an essential contribution to efficiency and quality improvement. Flexible machining centers aim to reduce production cycle times. The placing and clamping of several components on one fixture, and later removing them,is often a major time factor in the process. In order to be productive and efficient, it is important that procedures such as positioning, clamping, support and release be rapid, straight forward and safe. This is more important for larger workpieces with relatively brief processing intervals making semi-automatic or fully automatic clamping in one fixture a profitable option. Hydraulic positioning and clamping is an extremely reliable and efficient technique.


Time gain and accuracy

The major benefit of hydraulic workholding is the enormous time saved in clamping and unclamping the components. When we compare the time required for hydraulic clamping with that required for manual clamping the gain is no less than 90 to 95%. The use of Enerpac hydraulic workholding products reduces cycle times thereby increasing manufacturing capacity significantly and reducing costs. Equally as important as the time advantage is the positional accuracy of hydraulic clamping systems.The clamping forces are constant resulting in very precise positioning and clamping. This ensures identical processing procedures and guaranteed quality. Rejection rates due to distortion will be insignificant. A third advantage offered by hydraulic clamping is optimum use of clamping space due to compact standard components and the ability to clamp in manually inaccessible areas. This can increase the number of components that can be clamped and processed simultaneously on one fixture.


Basic principles

The basic design of any fixture using manual or hydraulic clamps is the same. The structure of the fixture must be sufficiently rigid and heavy to avoid deformities and vibrations due to machine movements and clamping forces. The unprocessed component must initially be positioned, supported and clamped only at three fixed points. When the workpiece is clamped on these three fixed points it’s position is statically determined. We can compare this with the stability of a table with three legs.


Three functions

Any workholding fixture must fulfil three basic functions:

a) position the component accurately.
b) support the component adequately.
c)clamp the component securely for machining.

During positioning, the workpiece is pressed onto fixed limit points and support surfaces. The workpiece must then be clamped in this position so that the cutting forces cannot displace it during machining. The workpiece may also need to be supported at other points to overcome distortion, vibration and torque. These forces can deform the workpiece and influence the accuracy of processing.
To overcome these forces the workpiece will be engaged by support cylinders with a light spring pressure (approx. 9N–15N.), after which they will be locked hydraulically. This will create support tailored to the workpiece. All three basic functions can be achieved with Enerpac workholding products. The three functions must be carried out in the proper sequence.This means that the appropriate cylinders will have to be activated at the correct times (sequentially). Normally this is achieved with mechanical sequence valves or solenoid operated directional valves fitted into the hydraulic system. Hydraulic pressure may be provided by a variety of pumps, such as manual pumps, air-hydraulic pumps and electro-hydraulic pumps.


The correct choice

When designing a hydraulic workholding system the correct choice of components is very important. Examples of things to consider: Is there enough space, is clamping along the edge appropriate or should it be internally? This requires each design to be considered on a case-by-case basis and there are times when the solution will involve a system with special components. This will also naturally lead you to question whether hydraulic clamping is justifiable. Enerpac has developed a software program specifically for this purpose that provides clear insight into the advantages and profit in a quantitative fashion. When a decision is made to develop a hydraulic clamping system a logical plan will provide the best chance of success. The plan should include the following choices.

  1. the clamping force and the clamping stroke of the cylinders;
  2. the power source;
  3. the control of the clamping system (accessories and valves)

The choice of clamp tooling as a function of the processing sequence in particular is extremely critical. The size and shape of the workpiece determine the various clamping points and hence the clamp selection.

Enerpac offers a complete range of hydraulic clamping products for this purpose. For each specific application the appropriate type of clamping cylinder, pump and control systems are available.

Hydraulic clamping of components for machining on conventional or pallet type machining centers has major advantages.


Standard components
 

Standard components such as swing cylinders, clamping cylinders, support cylinders, (for precise support of workpieces) and linear cylinders (for positioning the workpiece) may be employed in a hydraulic workholding system. Automatic coupling and uncoupling of the hydraulic system is used in many instances on pallet type FMS processing machines. Swing cylinders play an essential role in hydraulic clamping. They have been developed to enable the product to be inserted and removed freely. They employ a combination of rotary and linear movement.Rotation is achieved (left or right) by a helical groove that turns the piston and the clamping arm during the initial part of the clamping movement, the final part being vertical. Enerpac manufactures swing cylinders in many models and mounting options with clamping forces from 1.3kN to 37.8kN A patented design clamp arm transmits these forces from the swing clamp to the workpiece.

Where full access to the top face of a component is required edge clamps are available. One third of the linear force applied to the workpiece is directed downwards due to the design of the clamp. This guarantees safe & consistent clamping.

In cases where hydraulic pressure may not be applied during the machine cycle – for example on a mobile fixture or when there is increased risk of hose rupture – “ spring power cylinders’ may be employed. Disc springs provide the force in these cylinders. Hydraulic pressure compresses the disc springs and when released they exert the clamping force. The clamping force of a spring power cylinder depends upon the amount of spring compression.


Conclusion

There are many significant benefits with hydraulic clamping systems. The major advantage of hydraulic clamping is that it significantly reduces the load and unload times compared to conventional manual clamping. This results in higher capacity utilization on all types of machines. Hydraulic clamping also offers improved quality due to consistent and repeatable clamping forces being applied. Totally automatic operation is also possible with hydraulic clamping. Enerpac can also design and supply special production automation products if our standard range does not meet your requirements.

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