ISSUE #07

Support Cylinders

Selecting a Hydraulic Work Support
The basic selection of a work support for a fixture depends on two factors: determination of the support force required, and then from that, the size of support cylinder required.

Determine support force needed
A work support is required if a portion of a part would be moved or deformed by forces due to clamping or machining. See Chapter 6 for a discussion of how clamping force is calculated and the next paragraph regarding machining forces.

Determine work support size
As a rule of thumb, the capacity of a work support cylinder at the hydraulic pressure to be supplied to it should be at least twice the clamping force to be applied. (By “capacity” we mean the amount of opposing force the support cylinder is designed to withstand when in its locked state.) For many applications a 2:1 margin of support cylinder capacity over clamping force is sufficient to also absorb machining forces. In cases of extreme vibration or interrupted cuts, the margin of support cylinder capacity should be increased to 4:1.

The support cylinder selection process involves both the clamping and support cylinders, and four criteria that need to be satisfied:
  1. The necessary clamping force must be calculated.
  2. The machining forces acting on the support cylinder should be determined, if possible.
  3. The support cylinder capacity selected should be equal to the applied clamping force plus machining force plus a reasonable margin, or else at least twice the clamping force value.
  4. A common feed pressure should be determined for both clamp and support cylinders, otherwise an appropriate pressure control valve should be used.


The basic tool for cylinder selection is the pressure-force diagram. When a work support is to be used opposite a clamping force, the clamping and support cylinders are selected at the same time, using pressure-force diagrams for both types of cylinders to accomplish the tradeoffs between force, pressure, and cylinder rating (size). In some situations, it may be necessary to operate the clamping and supporting cylinders at different pressures in order to obtain the desired results.