October 22, 2009 ISSUE 216
Inquiries occasionally come to our web site seeking help in rebuilding a screw press. We offer the following guide.
The first, and easiest, items to check are the OEM components. These include the motor and gearbox, shaft coupling, air cylinder, air regulator, and possibly pillow
block and flange bearings. Maintenance mechanics are generally familiar with these, so little guidance is required. The OEM source and part identification is
included in the ownerâ€™s manual.
Rebuilding the rest of the press is focused on screw-to-screen clearance. This activity naturally falls into making sure the screen is round and properly positioned
in the frame of the press, and making sure the screw is round and centered in the screen.
Screens are either one piece cylinders, or cylinders made of two halves bolted to a pair of resistor bars.
One piece cylindrical screens are generally replaced if they have become twisted, beer-canned or extensively patched.
Two piece screens can become egg-shaped with the passage of time. To check this out, it is convenient to make a pattern from thin steel or cardboard, cut in the
shape of a half circle. The diameter of this circle is cut to the nominal diameter of the press, such as 16.00â€, 21.00â€, 24.00â€, etc.
With this pattern it will be easy to see if a screen half has bowed in or out. Such a condition is corrected using a hydraulic ram.
Screen halves can also become warped lengthwise. This is detected by placing the screen face down on a flat surface (the floor) and looking for warpage.
Corrections are made with a hydraulic ram.
Once the screens are straightened, the mounting of screens in the frame of the press is checked. Most presses have indexing rings in the B and C plates, while
some presses depend on the bolt holes in the resistor bars to establish a centered position. In the shop, we work off a laser beam centered in the holes in the A, B,
and C plates.
In the case of screw alignment, one of two situations occurs, depending if the screw is mounted in a hollow-bore gearbox, or if the screw is supported by
bearings at either end.
In the case of a hollow-bore gearbox, it is extremely unlikely that the gearbox will need to be moved in order to re-position the screw. However, the outboard support
bushing or bearing may need to be shifted. This need is evident if the screw has rubbed the screen.
On presses where the screw is coupled to the output shaft of the gearbox, the first step is to un-couple the screw. The screw is then centered in the screen. At the
drive end this is done by shimming the pillow block bearing up or down and by moving it sideways with jacking bolts. At the discharge end, the screw is centered
in the screen by moving the flanged thrust bearing, up, down, or sideways, with jacking bolts.
Once the screw is centered in the screen, then the gearbox is aligned to the screw. In this process the gearbox, not the screw, is the part that is moved.
Once the screen is centered in the press and the screw is centered in the screen, the screw-to-screen clearance is measured. The goal is to minimize this clearance
without having the screw rub the screen. If all the clearance readings are 1/16â€ or more, the diameter if the screw is built up by welding and then turned down in a
lathe. Should minor high spots occur causing the screw to rub the screen, the OD of the screw is hand-ground to minimize the interference.
In more severe cases of wear, some of the flights, especially at the discharge end of the screw, will have to be replaced. It is recommended that the screw be
straightened after major welding. Similarly, the bearing journal, cone sleeve, and seal surfaces of the screw should be restored. This work is done with the aid of a
It is important, at the least, to be sure that grease will flow through the grease lines that are used to lubricate the cone bushings. When the cone is removed from the
press, the insides of the cone bushings are inspected for wear.
If the screw is going to be removed from the press for any reason, be sure to have a replacement shaft seal on hand.