They also deliver stable, consistent force despite changes in temperature and wear on the tool. That's why engineers at Henrob Corp., Novi, Mich. (www.henrob.co.uk), chose the inertia design for its new Servo Rivet Setter.
It uses energy stored in a spinning roller screw, one with more mass than a typical roller screw, to move a plunger/punch that drives rivets. A pushrod connects the screw and plunger and a servomotor turns the screw via a belt drive. The motor produces relatively little torque, but can spin the screw at any rate from 0 to 5,000 rpm. The plunger advances when the screw spins clockwise and retracts when it spins counterclockwise. A steel block, or shaft coupler, attaches to the screw end and keeps the pushrod from spinning. This block has two slider blocks, or keys, which engage matching slots running the length of the actuator's front end. A shear pin connects the rod and shaft coupler and breaks if an overload becomes imminent, acting like a fuse and failing before real damage is done.
To set a rivet, the motor spins the flywheel to a user-determined speed while the plunger/punch moves toward the joint. As the plunger pushes the rivet into the material, the resistance of the rivet and friction slow the screw, bringing it to a stop when the rivet is flush in the joint.
The plunger spins as it retracts, storing energy. To prevent the actuator or plunger from slamming into the travel stop with too much force, the motor acts as a regenerative brake. The motor becomes a generator in braking mode, converting stored energy into electricity, which is channeled back to the servocontroller and stored in capacitors. The power is used the next time the motor accelerates the actuator.
The riveter motor is only sized to accelerate the actuator. It doesn't need the power required to press in a rivet (as in torque-based machines). This means it can use a simpler, lighter motor and transmission. And the device gets its power from speed, so it is much faster with shorter cycle times than torque-based machines. The inertial riveter also doesn't need closed-loop control and all its associated sensors and actuators to tell it when to stop pressing. Driving rivets depends only on actuator speed, so it only needs a single rpm sensor to confirm the speed requirement is being met. And velocity or rpm is a variable that can't wear out or drift (as force sensors can), so the power setting is always accurate and needs no calibration or tuning.