The most objective way to know whether a screw-barrel set is running in good condition is to take regular measurements. An accurately measured clearance value clarifies both maintenance planning and the decision to purchase a new set.
Required Measurement Equipment
- An internal diameter micrometer or a three-point bore gauge (for the barrel)
- An outer diameter micrometer (for the screw)
- A depth micrometer (for screw channel depth)
- A clean, oil-free measurement surface and adequate lighting
Measuring the Barrel Inner Diameter
After the barrel is removed, the inner surface should be cleaned and measured with an internal diameter micrometer at a minimum of 3 different zones (inlet, middle, outlet), taking readings from 2 different angles in each zone. The difference between zones shows which part of the barrel is wearing more; the feed zone and the area around the screw tip are usually the most worn zones.
Measuring the Screw Outer Diameter
The outer diameter of the screw flight backs should be measured with an outer diameter micrometer at the same zones as the barrel. If the screw has more than one flight (a multi-start screw), the crest diameter of each flight should be checked separately.
Calculating the Wear Clearance
The clearance value is calculated by subtracting the screw outer diameter from the barrel inner diameter and dividing by two: (Barrel Inner Diameter − Screw Outer Diameter) / 2. For a new set, this value is typically between one and two thousandths of the screw diameter; it increases as use progresses.
What Clearance Value Is Considered Critical?
The critical clearance value varies according to screw diameter, raw material type and machine type, so it is not correct to give a single universal number. However, a clearance that reaches 2-3 times the starting value is generally the threshold where capacity and quality losses become noticeable. It is recommended to obtain a reference value for your specific machine from the manufacturer or your technical consultant.
How Often Should Measurements Be Taken?
For lines processing abrasive raw materials, measuring every 3-6 months, and for standard raw materials, measuring during annual periodic maintenance, is sufficient to track the wear trend.
By sharing your measurement results, you can receive a free technical evaluation of your set's condition and any potential need for replacement.
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