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TribSheet # 202
The Twist Compression Test is used to evaluate lubricants and die materials for application in metal working processes. This test measures the transmitted torque between a rotating annular cylinder and a lubricated flat sheet specimen. The 25mm (1") annular cylinder rotates at 9 rpm or 10mm/s (25 inches/minute) and pressure is set to best simulate the process. The coefficient of friction is calculated from the ratio of transmitted torque to applied pressure.
 
The TCT is best used as a comparative rather than absolute test. As with all tribological evaluations the influence of uncontrolled (and in many cases unknown) may cause unexplained shifts in data. These shifts may be due to seasonal differences or very minor tooling variations. Consequently, it is advisable to include a reference lubricant and material for all evaluations.
 
LUBRICANT PERFORMANCE INDICATORS IN THE TWIST COMPRESSION TEST
 
Friction - Indicates the effectiveness of the lubricant at reducing the interfacial shear stress. Interfacial shear prevents movement of sheet material into a die or distribution of material over a punch.
Time to Breakdown - Indicates ability of the lubricant to prevent adhesion between the tool and the workpiece. This is a function of the lubricant film strength and additive action.
Pickup/Galling - The nature of metal transfer in the TCT has been found to be an excellent indicator of tool wear.
 
CRITICAL FEATURES
The TribSys Twist Compression Test has several features that are critical for the successful evaluation of lubricants or die materials.
 
Self centering tooling to ensure concentricity of annular tool and sheet specimen holder
 
Self aligning sheet holder on a high capacity spherical bearing to ensure parallelism of tool and workpiece
 
High capacity ultra-low friction bearing for low losses
 
Engineered for low noise and maximum signal to noise ratio
What is die wear?
Die wear, simply put, is a change in a die surface that may affect die performance.
 
Why should I worry about die wear?
Die wear, like death and taxes, is unavoidable. But can be controlled so that your dies live long and productive lives.
 
What can I do about die wear?
Some types of wear occur gradually and by monitoring die wear, die maintenance and replacement can be predicted. Other types of die wear occur rapidly and are unpredictable. These types of wear can put a process out of control or cause die breakage. The best defense against wear is to know the types of wear and how to deal with each.
 
There are several types of wear including:
 
Adhesive Wear
Caused by: Cold welding
Results in: Pickup, scoring, etc.
 
Abrasive Wear
Caused by: Hard surfaces or particles
Results in: Metal removal
 
Fatigue Wear
Caused by: Repeat loading
Results in: Micro-cracking, spalling
 
Chemical Wear
Caused by: Chemical Reaction
Results in: Reaction Products
 
Mixed Wear
Caused by: Combinations of above
Results in: scoring, scuffing, galling
 
Adhesive wear is actually a build up of workpiece material on the die known as pickup. These photographs show pickup on a tube bending mandrel and the resulting scoring on the inside of the bent tube. The scoring increased drag and caused the tube to split during the bending process.
 
Selecting the proper die material/coating and choosing a lubricant that provides protection against metal-to-metal contact can avoid this type of wear.


All information presented on this web site is copyright   TribSys Inc. 2005.