June 22, 2022

Frequently we are asked about the service life to be expected from the components of a screw press. This is highly dependent on the application in which the press is operating.

A third of our work is in international projects, and it happens that those applications have very low maintenance. For example, we were in a plant in Indonesia where the press has been running 24/7 for over seven years, making boiler fuel from coffee grounds. We opened up the press and the screw and cone bushings were literally like new. The screen had some torn areas, but that came from when a large piece of tramp metal had entered the press.

It is the same story with presses making alginate, pectin, xanthan gum, soybean protein concentrate, and both cattle feed and pectin peel from citrus waste. We usually get zero spare parts sales from these customers.

We checked with engineers here in the office about service life.

The first replied that the applications with the highest wear are fiberglass waste from insulation factories and sludge from plastics recyclers. In factories which recycle post-consumer plastic bottles our presses dewater the sludge. The sludge is dirt and paper fiber, but it has a lot of sand and glass, plus some tramp metal. In fiberglass, screws and screens last 9 to 15 months.  In plastic recycling, they get 12 to 18 months.

Another engineer mentioned eggshells (we have sold almost two hundred screw presses which are separating protein liquid from eggshells) and swarf. Swarf is the metal shavings from machining operations; we squeeze out the coolant liquid which is used in the lathes and mills. That one is a tiny market. The screens are the problem for both eggshells and swarf. A wedgewire screen in eggshells will go two years, while a perforated liner screen lasts six months. In swarf we see severe screw wear.

Rebuilding a screw costs about half of the cost of a replacement screw.

A few years ago we started started something new, using HVOF on wear components in highly abrasive applications. The High Velocity Oxygen Fuel process is used to provide extra wear resistance to the inside of the last and first third length of reversible screens, as well as the journal and seal areas of shafts of the screws. Depending on the application, we have it applied to screw flights, on top of the regular hardsurfacing. It looks like we can expect a 50% increase in service life.

HVOF is the first time we have been able to harden our screens. It is applied to the first and last third because the discharge end of a screen is the first to wear out.

Air cylinders frequently have a short service life of only a few years. The small ones (4ʺ, 5ʺ and some 6ʺ cylinders) are not rebuildable and must be replaced. Overhaul kits are available for the larger ones.

Our gearboxes are selected for Class II service — continuous duty, mild shock load. So they are expected to run 15 to 25 years.

We frequently consult with customers who are not achieving these results.

ISSUE #343