Using steel balls, oil, PVC paste and grease in fixture design

Often in fixtures we want some extra features like multiple clamping, uniform loading to a complicated surface, high clamping forces and the most important within a limited space. The steel balls, oil, PVC paste (or soft plastic material) and grease have a major advantage: all of them are capable for transferring a applying force to one or multiple direction without the aid of complicated mechanisms. The above parts and materials can be used direct or indirect relative to workpiece contact surface. Direct usage simply means that the element contacts the workpiece, and indirect that the element press, squeezes, pushes or expand an intermediate part. The steel balls belong to both categories and the rest of materials to the second one.
The steel balls are easy to get and very useful. Every bearing distributor can supply various sizes and tolerances depending on our application. The advantage of the perfect sphere is that it can redirect a force which is applied on the sphere’s surface. By providing an appropriate seat the steel ball is guided to a certain direction and the same will occur with the force. Numerous steel balls with a very small size enclosed in a container react like the oil. When the steel balls are pressed with a conical shaped ram are expanding towards the direction of the contact jaw or pin. The steel balls are broadly used within internal clamps with one or multiple contacts, compensators and equalizers (see product t-1108 double contact pin equalizer) and mandrels.

Whilst in indirect appliance the design is simple, in the direct one the steel balls must be limited in order not to be lost during unclamping. Providing holes (steel balls guides) with peen edges and special plugs with o-rings are the most common solutions. When applying high loads remember the produced Hertzian loads at both surfaces (steel balls and workpiece or intermediate contact).

Typical usage of oil, PVC paste and grease is with an internal clamp or with a elastic sleeve mandrel. In these configurations when tightening a screw the element is compressed which, while expanding a thin-walled sleeve, holds the work clamped in position. Air cylinder or other actuation mechanisms are also used in order to increase the pressure force. In such configurations lap fitting of the ram, fine pitch screws and sealing equipment are used to prevent leaking of the element. Also passages with screw plugs must be placed opposite to the element entrance hole for removal of air while filling. The fixture components which press or been pressed by the element must be design to prevent leaking, especially with mandrel configurations due to high pressures.