screw pump

SCREW PUMP

1. Introduction

Screw pumps are one of the most reliable pumps for hydraulic applications. Especially these pumps are employed where quiet operation is essential.

2. Construction

The screw pump is an axial flow, positive displacement type gear pump. A two-rotor screw pump with helical gears and its cross-sectional view are shown in Figs.4.9(a) and (b). 

These pumps have (two or more) meshing screws to develop the required pressure. These screws mesh to form a fluid tight seal between the screw recess and housing wall. The only moving parts in these pumps are screws.

In the two-rotor screw pump, the driving screws and driven screws are connected by means of timing gears. The timing gears are provided to maintain a running clearance between the two meshing screws.

3. Operation

The liquid is introduced at the two ends and discharged at the centre, as shown in Fig.4.9(b). The pumping action comes from the 'sealed chamber'. The sealed chamber formed by the contact of the two gears at the intersection of their addenda and by the small clearance between the screws and the pump housing. This working is similar to a nut moving along a threaded rod when the rod is rotated. In these pumps, it should be noted that the liquid does not rotate but moves linearly. Thus the liquid moves forward along the axis with the rotation of the screw and is discharged to the outlet port.

4. Advantages of Screw Pumps

Some of the noteworthy merits and demerits of screw pumps are presented in Table 4.5. 

Table 4.5. Advantages and disadvantages of screw pumps

Advantages

(i) Self-priming.

(ii) Most reliable.

(iii) Give uniform pressure with negligible pulsations.

(iv) Very quiet, because of rolling action of the screw spindles.

(v) Do not require check valves.

(vi) Produce relatively long wear on moving parts.

(vii) Can handle liquids containing vapours and gases.

(viii) Exceptionally long life.

Disadvantages

(i) These pumps are bulky and heavy.

(ii) It is difficult to manufacture the screw profile to maintain close tolerance.

 (iii) Pressure rating of the screw pump is viscosity dependent. So pumping of higher viscosity oil may decrease the pump efficiency.

(iv) Overall volumetric and mechanical efficiency is relatively low.

5. Range

Screw pumps are available with capacities upto 190 Lps and the discharge pressures upto 250 bar. Also these pumps can operate at very high speeds (upto 3500 rpm).

Note

From the above discussion, the following three advantages of gear-type pumps are evident.

1. Gear type pumps have fewer moving parts.

2. They exhibit simplicity in design and construction.

3. They are expensive.