Maintain Proper Belt Tension for Better Abrasive Performance.

Have you over-tightened your belts? Improper belt tension can prematurely wear out sanding machines or damage work pieces.

WHEN CONSULTING WITH CUSTOMERS, abrasives suppliers place a great deal of emphasis on choosing the best abrasive product for the job. To put that product to its best use, however, it's just as important to select and set the correct ma chine parameters. One of the most over looked aspects of wood sanding operations is maintaining proper tension on the coat ed abrasive belt once it is mounted on the sanding machine.

In basic terms, the amount of tension on the coated abrasive belt should be just enough to keep the belt from slipping on the drive wheel, and to keep the belt from tracking off the machine.

Some sanding machines have fixed belt tension. For example, the tension in portable sanders is derived from a factory-set loaded spring, which is not readily adjustable.

However, most sanding machines have some type of adjustable tensioning device. These include use of weights, screw-type mechanisms or pneumatic cylinders.

On screw-adjusted tension devices, such as those found on edge sanders and some mould sanders, it's often difficult to determine the proper amount of tensioning unless the operator is experienced with the machine. As a general rule, the belt should make five or more full rotations after the machine is turned off. If the belt comes to a stop very quickly, it's likely that the tension is too high.

Pneumatic cylinder designs are the preferred type of adjustable device. They compensate for stretching of the abrasive belts, provide a range of delicate adjustment and can be rapidly released and applied for quick belt changes.

Pneumatic tension can be monitored by using an air gauge installed on the sander. These gauges should be monitored as well, because if they are not properly maintained, they will give incorrect readings. Air pressure gauges are relatively inexpensive, and should be replaced if broken.

Most wood sanding applications (such as mould sanding using narrow cloth belts) re quire a belt tension of approximately 40 psi. However, there are some special exceptions. For example, abrasive planing operations using coarse grits typically demand belt tension in the range of 70 to 90 psi.

Intermediate wide belt sanding (grits 60 to 220) normally uses a tension in the range of 60 to 75 psi for cloth belts. On these applications, tension for paper belts should be set at about 20 to 25 per cent less than that for cloth belts. This is because cloth belts will stretch slightly during use, while paper belts will not. In any case, the wide belt machine manufacturer should be consulted for specific recommendations regarding air tension settings.

On drawer sanders -- a specialized machine for sanding the inside lip of a drawer front where it joins the sides -- it's best to use the least amount of belt tension possible when installing a new belt. This means that you should be able to easily rotate the belt by hand with the machine turned off. After running the sander for 10 minutes, it may be necessary to increase the tension as the belt stretches slightly.

The Dangers of Over-tightening

Because the goal is to keep the belt from slipping, users sometimes err on the side of using too much belt tension. Several problems can result from excessive belt tension. They include:

* Premature bearing wear

* Excessive wear of the rubber contact drum

* Excessive wear of graphite covers on platens

* Excessive wear of mould blocks

Excessive belt tension changes the characteristics of a rubber contact drum; it makes a soft wheel act harder and de creases the effectiveness of the serrations.

Excessive belt tension can also lead to belt failure. It can cause the belt joint to mark the workpiece and can cause tracking problems.

But the leading problem caused by excessive belt tension is the generation of heat. One recent study showed that an increase in belt tension from 40 to 60 psi caused the sanding heat at the area of con tact to jump by 100F. Excessive heat often leads to premature loading of the belt and wear on flexible belts.

Consult with your abrasives supplier and machine manufacturer to ensure that you are using the proper amount of belt tension. Maintained correctly belt tensioning can maximize abrasive belt life, provide the best possible finish and extend the life of the sanding machine components.

Doug Washer is Regional Product Engineer for Norton Company.