Belt Tips for Better Metal Sanding and Grinding

October 1, 2018

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Abrasive metal-finishing belts of all sizes require tension to operate. Although some machines use spring pressure to create belt tension, and some wide-belt sheet- and coil-polishing equipment employ hydraulic belt-tensioning cylinders, the vast majority of belt-sanding and -grinding machines use pneumatic belt-tensioning cylinders.

For quality and productivity when belt sanding or grinding workpieces, the importance of appropriate abrasive belt tension cannot be over-emphasized. In many cases, problems relate directly to the amount of tension applied to belts. However, problems related to high or low belt tension often are caused by something unrelated to the tensioning function itself. How can you determine the direct cause, and how can you fix it?

As a general rule, set belt tension at the point where the abrasive belt does not slip on the drive wheel (narrow belts) or roll (wide belts) when it engages during the sanding/grinding process. But where is that point? And, even if that point is found, it can change easily as the abrasive belt wears and more friction develops between the part and the face of the belt as the operator applies more pressure. A complicating factor: Machines using abrasive belts feature a significant amount of design variation, which often requires differing amounts of tension. Following are effects of low and high belt tension, as well as possible causes.

Low Belt Tension

belt sanding and grinding, importance of appropriate abrasive belt tension

For quality and productivity when belt sanding or grinding, the importance of appropriate abrasive belt tension cannot be over-emphasized.

Abrasive belt tracking problems. Here, the belt randomly tracks off of the head in either direction due to a poor grip on the tracking wheel/roll. Do not confuse this with a belt tracking off in the same direction, which may indicate wear on the tracking wheel/roll or tracking-mechanism parts, or side-to-side length variation on the belt.

Printing on the back of the belt shows a high degree of wear. This indicates a belt slipping on the drive roll, which will wear away the print.

Parts being processed have “skip” marks where the belt has slowed or stopped during grinding/sanding. This can occur when a belt randomly slips on the drive roll for short periods of time, which typically occurs when the head encounters higher workload.

Short-term high-pitched whining sound with the grinding/sanding head in start-up mode and/or when processing parts. This may be a symptom of the belt slipping on the drive roll and likely has significantly less tension as compared to the above scenario.

Belts folding over on themselves and typically breaking. This occurs under low belt-tension conditions; however, it also can result from wear on any of the rolls that the belt runs over, or a combination of these two conditions.

Belts breaking at the joint where the joining tape is on the back side of the belt. This can be caused by the belt slipping on the drive drum, causing excessive wear to the joining tape.

Reduced belt life. This can result from abrasive grain not breaking down and resharpening due to a reduced amount of pressure/force being applied between the workpiece and the grain particles when belt slippage takes place.

High Belt Tension

Abrasive belt tracking problems. Here, high tension overwhelms the tracking cylinder, which typically is much smaller than tension cylinders.
Premature bearing wear on the head’s drive, tracking, and/or idler rolls. The higher the belt tension on any type of grinding/sanding equipment, the more wear on all related bearings. This results from the over-tensioned abrasive belt pulling the OD of the rolls or roll shafts against the outer race of the bearings at higher pressures than recommended. This creates higher pressure between the ball/rollers and the inner and outer bearing races, resulting in dangerously high bearing temperatures, followed by bearing failures. On sanding operations that develop combustible particulate, such failures can become fire hazards.
Belt breakage. High belt tension reduces the belts ability to absorb high or fluctuating shock loads. Breakages will occur in the body of the belt or in the joining area.
Increased wear on all rolls within the head. This type of wear typically will be located where the abrasive belt tracks back and forth toward the outside edges of the rolls. However, with higher belt tension wear increases excessively.

Watch Pressure Gauges and Regulators

Air pressure gauges and regulators negatively affect grinder performance

Faulty air-pressure gauges and regulators, as well as air-supply leaks, all can negatively affect grinder performance. These facets of the air-delivery systems should be checked to ensure proper operation.

In most cases, use the OEM’s recommended belt-tension gauge pressure.

For steel or hard-rubber contact roll heads, use 20 to 25 lb. of pressure per linear inch of belt width.
For combination heads and platen heads, use 15 to 20 lb. of pressure per linear inch of belt width.

However, should problematic conditions exist even with the gauge pressure set at its recommended psi, check the air gauge for accuracy. Inaccurate gauges have been directly linked to issues that cause expensive, unscheduled downtime.

To check the air gauge for accuracy, have maintenance personnel convert a standard air regulator to a working measurement tool. Here, the regulator must have two output ports and one inlet port. Fit the inlet port with an air plug that matches the coupler on the air line closest to the grinding/sanding equipment. Fit one of the outlet ports with a quality air gauge in good condition, preferably a glycol-filled gauge that is much less likely to lose its accuracy or be affected by environmental conditions. The second outlet port should be fitted to accept the threads of the belt-tension air gauge.

With such a tool available, thread the air gauge to be tested into its proper outlet port and plug the air supply line into the regulator. Check the air pressure readings between the ‘good’ gauge and the ‘suspect’ gauge at various psi settings to determine accuracy. When not in use, store the measurement tool in a secure place where it cannot be tampered with or disassembled.

Besides belt-tension air gauges, their related air regulators also are key to the condition of grinding/sanding equipment. Leaking or poorly operating air regulators have a direct effect on the air supply to the tension cylinders. Sometimes they can cause high belt tension—unregulated air routing to tension cylinders—or low tension due to insufficient air-pressure supply. Periodically check all regulators to ensure good working order.

Air Leaks Affect Belt Tension

Air leaks also can play a role in abrasive-belt tension problems. Leaks can occur in the air-supply system as well as in the seals of the tension cylinders themselves. Problems caused by these leaks, and the cost of consumed compressed air due to leaks, are commonly underestimated. All air leaks should be repaired, and it is advisable to conduct an air audit throughout your facility. This is a relatively inexpensive way to make the compressed-air system more efficient, which in turn will increase productivity and improve any processes that rely on compressed air. MF

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