Combination clutch-slip device.

* protects operators

* protects drive gears

When Bosch created a new line of professional hedge trimmers for the vast hedgerows that populate the UK and European countrysides, it needed a tool that was both powerful and safe. One of the early design challenges the company faced was providing a clutch mechanism that would disengage the oscillating blade if the operator took his/her hands off the grips. When engaged, the clutch had to be able to supply the torque needed to cut through the occasional thick branch without slipping. At the same time, the mechanism had to protect the drive gears from damage if the trimmer encountered something tougher--like a chain link fence. And, it had to be durable, lightweight and compact.

To help meet this complex series of performance characteristics, Bosch turned to Reell Precision Mfg., for its expertise in small-package torque control. Working through Reell's European facility in the Netherlands, they were able to design and manufacture a compact mechanical clutch based on the company's wrap spring clutch technology. Also built into the mechanism was a constant-torque slip device, based on the company's patented clip and shaft technology. One technology provided a method for disengaging the cutting blade from the motor to improve operator safety--the other sets an upper torque limit to protect the motor gears in the event of striking an obstacle.

"This particular hedge trimmer is designed for professional landscapers and is a very powerful tool that generates more than 3/4 hp," says John Kossett, project engineer at Reell. "The motor constantly spins at about 25,000 rpm and is geared down to turn the clutch at just 1500 rpm. Acting like an on-off switch, the clutch engages the blades when the operator grips the handles with both hands. If the trimmer hits something it can't cut through, the blades will stall instantly. However, since the armature of the motor is spinning at 25,000 rpm, it has a lot of kinetic energy in the form of momentum. If there is no 'give' anywhere in the system the small drive gear tends to fail. The built-in slip device allows the kinetic energy to be dissipated over about 1/2 revolution of the clutch, safely slowing and stalling the motor."

The design offers a safe and ergonomic trimmer that delivers high power in a compact, lightweight package. It is the first time that Reell's wrap spring clutch has been used to control 80 Nm (7008 lb-in.) of torque in such a small device. The integral slip device was derived from constant-torque hinges, and it is the first time the company's clip technology has been implemented in such a high torque range, according to Kossett. While the previous design took 1.5 sec for the motor to stop, this version stops in less than 0.1 sec to reduce the possibility of operator contact with the moving cutters.

In a wrap spring clutch, a coil spring rotates with the drive motor. The output shaft is located down the center of the spring's coil. If the tail of the spring is moved opposite to the direction of its windings, the coil expands and disengages the output shaft. If the tail of the spring is allowed to relax, the coil shrinks and grabs the shaft, transferring the rotation to the output. After Reell designed the clutch-slip device, Bosch asked them to manufacture the parts as well. The two companies worked together to develop a semi-automated production line in the Netherlands, with 100% functional testing of each completed assembly.

Reell Precision Mfg., St Paul., MN Circle 152, or www.rsleads.com/410df-152

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