Q&A
Ask the Expert Your aquafeed processing questions answered
Q: Did you see pictures of screw profile? Is this ok? I don’t think there is much wear. Ah, maintenance! Screw and barrel sleeve wear can catch you unaware and will inevitably cause more grief on those products which are trickier to run, such as low-fat small diameter sinking feeds, with some raw materials more sensitive to the reduced pumping and increased shear caused by wear. As a rule of thumb on new parts for a nominal 10 t/h single screw, a gap of no more than 2 mm between head and screw would be appropriate (the gap being proportional to extruder size) and by the time that gap has hit 4 mm, you could be expecting some issues to pop up, such as surging, excess heat and general lack of pellet uniformity. When wear is more than this, you may start to experience some back feeding as well, particularly on smaller machines. Refined starches and low wheat formulations are likely to show up any wear related problems earlier. But don’t hang your hat on a fixed wear diameter measurement – read on. Using a shaft support bush is always preferable, as without it deviation caused by radial load will create excessive forging wear (metal on metal contact), particularly on the final head and screw, not to mention reducing life of the main thrust bearing, as well as reduced bearing seal life, leading
to oil leakage from the bearing housing. If your shaft support is worn to the point it is allowing screw on head clash, then this is a priority fix – a replacement bush is going to set you back a lot less than replacing screws! Clearly, the shaft needs to be aligned correctly in the first place, using a clock/dial indicator to ensure minimal runout (deviation from center). Don’t overtighten nuts when pulling up a shaft, or your bearing will run hot with excessive load. This will require experience to perform, using supplier specification. Perform regular checks on the shaft locking mechanism. If a locking ring with retaining ring/tab washer is used, ensure the tab washer is intact and tabs are in place. Lock nuts can loosen and tabs move/break over time, resulting in excessive shaft play and premature wear. There is little point in my providing a fixed tolerance for bearing endplay or shaft runout, as this will depend on specification from the equipment provider and length of the extruder shaft. Tolerance in the shaft support bush and head support should take into account the amount of runout in order to avoid undue stress on the shaft from moment load in the shaft alignment. If you have a twin screw and notice what appears to be forging wear marks running down the face of
Peter Hutchinson is a Technical Editor at Aquafeed.com, the owner – director of ENH Ltd., New Zealand, and an aquaculture feed consultant. Send your questions to: pete@aquafeed.com
the flight, act immediately to check for shaft timing, thrust bearings or damaged/twisted shafts. Pinning a time frame on expected life for screws is not practical either, so I’m not going to attempt to provide a guide here. It depends on the type and hardening process of steel used, raw materials processed and operating conditions. I’ve come across screws flogged out in a couple of months by incorrect operation or utilizing sand loaded fishmeals for example, whereas the same screws may have lasted years under ideal conditions. The difficulty in providing a fixed measurement limit for screw and head wear is that it is so much
Aquafeed: Advances in Processing & Formulation Vol 12 Issue 2 2020
