TechArchive

This article was originally written in the period 1995-2000

The selection of the right pump for the job is primarily a matter of matching the required performance range against the capability of each product type, as shown in the accompanying charts.

With over one million pump units sold into more than 35 countries, ASF Thomas supplies a broad product range of air compressors and vacuum pumps that fall into a number of major product groups. The operating principle of each product group is described and illustrated. Any special features which may influence its suitability or unsuitability for a particular application are also mentioned.

A peristaltic pump is required where integrity of the pumped medium is paramount and no leakage can be tolerated. This is achieved by enclosing the medium in a continuous tube which is mounted in a cylindrical casing. Within the cylinder is an eccentrically-mounted shaft, carrying a set of freely rotating rollers fixed on a pair of plates and these rollers are in near-continuous contact with the tube.

As the pump shaft rotates, the eccentric shaft initially causes the pumped medium in the tube to be t rapped against the casing and between adjacent rollers. It is then moved around the cylindrical casing until it exits the pump as the rollers release it. Flow rates can be closely controlled and fiarly aggressive materials can be handled.

In a diaphragm pump, reciprocating motion in a connecting rod is produced by an eccentric bearing on the pump’s drive shaft. Attached to the con rod is a diaphragm which is flexed in a closed chamber, alternately compressing and expanding the contents of the chamber, with inlet and outlet flapper valves controlling the direction of flow.

Whilst similar in concept to a piston pump, there is no sliding seal between moving parts, so integrity of the medium being pumped is preserved, making this the preferred pump when contamination is to be avoided. The chamber is fully exhausted during compression and while the stroke is much shorter than a piston pump, similar vacuum levels are achieved.

On a rotary vane pump, the pumping action is produced by a set of flat vanes fitted on a rotor eccentrically mounted in a cylindrical casing, thus producing a set of unequal swept volumes between vanes. As the rotor rotates on the pump shaft, these swept volumes gradually increase to a maximum, then decreases, with each cycle.

Inlet and outlet ports are carefully positioned in the casing so that the pumped medium is drawn into the expanding swept volumes during the suction half of the cycle, then compressed in the contracting swept volumes during the discharge half. This action produces relatively pulse-free flows from a compact, vibration-free unit.

WOB-L piston pumps operate on the same principle as conventional piston pumps, but with one essential difference: there is no bearing between piston head and connecting rod. This eliminates the need for piston rings. Instead, a rugged, flexible cup is fitted to the piston head.

With no small end bearing, the balanced eccentric cam on the pump shaft causes the integral rod and piston to wobble in operation. Resistance by the pumped medium during the compression stroke expands the cup during the compression stroke, compensating for the wobble and providing a highly efficient seal for consistent performance.

• ASF Thomas
• Tel: 01420 544184
• Fax: 01420 544183
• Contact: Warren Bease, Managing Director