Time and again, screw pump parts suffer wear before their time and the operator, not knowing better, assumes that a production fault is responsible. Usually, however, a more detailed investigation reveals that the cause of the shortened lifespan lies in the incorrect operation or handling of the pump parts or machine. It is generally very easy to detect the background for this wear behaviour and any associated problems by considering certain wear characteristics.
The main causes of wear and their effect on properties are therefore explained extensively using the following diagrams.
Furthermore, the table at the end provides clear information on assessing wear indicators and dealing with problems.
A screw pump can only work reliably if there is adequate and appropriate lubrication. This lubricant produces sliding friction of the rotor in the stator. It is similar, for example, to the phenomenon of aquaplaning with tyres on wet roads. If this lubricating film is damaged, high temperatures are reached and the elastomere wears within a very short time. Even if dry running lasts only a few seconds with new parts, the effect resembles that of a jump start on a new tyre, dramatically affecting length of service.
It is possible for quantities of material to pass through the pump which are dry inside, if mixing is inadequate or in the case of difficult materials. The lumps are much the same as lumps of cocoa floating on top of the milk if the drink has not been stirred sufficiently. Since these lumps are partially squashed between rotor und inside of stator, some degree of dry running ensues which interrupts the lubrication film between rotor and stator, with severe, detrimental effect on service life.
Pressure stability, material flow and starting torque of the pump are important factors which have to be suited to the grain size, grain form and consistency. Please ask for advice in order to find the best solution.
|D4 1/4||For smallest output, Trowellable Render (12 l/min)|
|D4 1/2||see D4 1/4 L., special plasters (15 l/min)|
|D6-3||lime cement plasters, gypsum plasters (25 l/min)|
|D5-2,5||same material as D6-3, but more output (27 l/min)|
|D7-2,5||same material as D6-3, but more output (30 l/min)|
|D8-2||Groove Grated plasters (50 l/min)|
Stator and rotor are designed to fit together perfectly. We are happy to mark the rotor heads of the pump with the same colour as the corresponding stators, to prevent any confusion. Acidic lubricants are not to be used in the assembly. We recommend our rotor pressing machine in combination with our special assembly spray.
This spray has the following advantages:
- Easy assembly without needing high strength
- Long lasting affectivity
- Clean, straightforward handling
- High yield
- Rotors and Stators don not stick together when assembly as they do, for example when soap is used
- Environmentally friendly
1. Spray for about 5 sec. into the inlet and outlet sides of the stator (making sure that the spray cannot escape unhindered through the stator)
2. Wet the Rotor briefly
3. Screw the Rotor into the Stator
4. Even after waiting approx. 10 days, the starting torque for the assembly pump only increases by an insignificant amount
As can be seen clearly in the enclosed diagram (start-up behaviour of a screw pump), the initial break-away torque is far higher than the nominal torque rating. If there are any hindrances to flow, the max. break-away torque will be 20-30 times as high. The rotor heads are not designed to withstand such loads.
The head breaks. Material faults in the grain of the failure surface are very easy to detect and are very rarely the reason why a head fails. The most common cause is insufficient cleaning of the pump, material hardens and the head breaks when the pump is switched on again. Storage of the assembled pump means that the break-away torque can in some cases double after standing for only 4 days, depending on the pump type. Use of unsuitable assembly aids can mean this statistic rises considerably.
The rotor of an intact eccentric screw pump must be far enough in the stator to be exactly aligned with the end of the stator. Correct positioning of a rotor ensures that all sealing edges have an optimal clamping ability when there is the highest possible conveying chamber volume. With adjustable stators, the pressure required can be accommodated by tightening the clamp to maintain the clamping ability of the sealing edges. The pressure required is regulated by the pump.
However, contrary to instructions issued in the past, care should be taken when setting the pump pressure for adjustable stators, that the clamps are tightened to the same extent along the entire length of the stator. Otherwise, in the case of conical stator tightening, the clamping ability of the sealing edges will be irregular, i.e. some individual areas will be too tight, whilst others are not tight enough. This leads to premature wearing of those sealing edges which are too tight and can result in mortar flowing backwards