Simple torque overload protection

Paul Boughton

Power tools, such as drills, saws and hedge trimmers, need to integrate some sort of torque overload protection to prevent the motor from stalling when an overload condition or jam occurs. This safeguard also helps eliminate the possibility of the motor burning out and prevent damage to the drill bit, if it’s a power drill, and drive gears.

Ultimately, overload protection is essential to ensure the lifetime of the power tool and maintain the manufacturer’s reputation for supplying quality products to a high standard. It can also protect the equipment users from potentially harmful injuries should the power tool fail.

There are, of course, conventional engineering solutions for protection of the motor, gear teeth and other drive components during a torque overload condition, but they are usually costly, complicated or both. An electronic approach to solving toque overload problems could be to use a current sensor to monitor near stall current conditions. Quite a complex solution, this method can be expensive and adds considerable weight to the final product. An alternative mechanical approach could be to use a clutch; however, this method is also expensive, weighty and increases the size of the overall assembly.

Building on the knowledge and experience gained in several successful designs already deployed within the automotive industry, engineers from Saint-Gobain Performance Plastics Rencol have created an in-line torque slip solution. Quick and easy to assemble, the new design helps reduce weight and save space of the overall assembly – two key advantages, when you consider that almost all power tools are either hand-held or portable.

Tolerance rings are deceptively simple in concept, inexpensive, compact and easy to use; although a high degree of expertise is involved in their design and manufacture. Essentially, they are radially sprung steel rings that are designed to be press fitted between two mating annular components to create an interference fit. The rings can be designed to provide either a rigid joint, such as for a stator mount in an electric motor, or as in this application to slip at a pre-determined torque level – from 100Nm down to 2Nm.

The specially designed tolerance ring is inserted between the gear and the hub to create an interference fit. Since the parts are rigidly connected through the ring, a zero backlash system is achieved. At a pre-determined torque level, when the load reaches a point at which damage would otherwise occur, the tolerance ring will flex and slip. As soon as the overload condition is removed, the interference fit is re-established allowing the power tool to continue normal operation. Key to this particular design is that overload can occur many times – up to 1000, 360-degree slips can be accommodated – without adversely affecting operation.

However, tolerance rings can be used in other applications besides torque overload. Others include bearing retention in as-cast, as-pressed or as-moulded housings, eliminating the close tolerance machining of the bearing bore. Tolerance rings can also be implemented to compensate for differential thermal expansion in dissimilar materials, for example in alloy or plastic housings, and can also tolerate high levels of vibration. Operational speeds in excess of 20,000rpm and temperatures up to 150°C can be readily accommodated with no loss of bearing retention.

Saint-Gobain Performance Plastics Rencol 

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