Imagine a disc-shaped part, such as a bicycle wheel, with a weight taped to the rim. When the bike is lifted off the ground, the wheel will rotate and come to rest with the weight at the bottom. If you were to spin the wheel, the bike would shake as the wheel tried to rotate about the center of the wheel’s mass, which is no longer located at its axle. The problem can be corrected by removing the weight or by adding an equal weight directly opposite from the unbalancing weight.
This is the purpose of equipment designed and built by Hines Industries Inc. of Ann Arbor, MI. The company makes fully and semiautomatic machinery for balancing rotating parts, such as propellers, impellers, armatures, pulleys, mower blades and turbo chargers. For these parts to operate optimally, they must be balanced. Hines Industries’ machines check the mass distribution of a rotating part and, if necessary, adjust it to ensure that the part doesn’t wobble or vibrate during operation.
Weight can be removed from a part by drilling, surface grinding, milling or punching, depending on the part and the material it’s made from. Weight can be added by welding, applying epoxy and insertion.
One of the company’s latest machines is a fully automatic system to balance drums for automatic transmissions. The transmission drum connects the central or sun gear to the exterior half of the clutch pack. Manual transmissions do not require these drums because manually operated gears slide along shafts to engage or disengage. In automatic transmissions, gears do not move other than to spin, so the drum is needed to engage and disengage the gears.
The transmission drum is constructed with an open seam. This allows the diameter of the drum to be controlled by a steel band with a friction-generating substance on the inside surface. The band is anchored at one end and connected to a hydraulic servo mechanism at the other. Tightening and loosening the band affects the engagement of the drum half of the clutch pack with the planet gear the other half of the clutch pack. In this way, an automatic transmission is able to change gears according to engine speed.
The drums, which are machined from aluminum, are delivered to the fully enclosed machine via conveyor. A robot picks up the drum and places it on the balancing instrument. The instrument spins the drum, measures the forces acting on it, and determines where to remove material and how much material to remove.
Next, the robot places the drum in a fixture below a servo press from Promess Inc. The press is equipped with a punch 3 millimeters in diameter. The fixture rotates the drum to the precise spot where material must be removed. Then, the press punches a hole in the drum. Depending on how much material must be removed, the machine might rotate the drum and punch several holes. Next, the robot removes the drum and places it back in the balancer for a recheck. If the part is good, it will be transferred to the next station. If it needs additional weight removed, it will return to the punching station.
The press isn’t programmed to go to a particular force, but rather a position—specifically, the exact point at which the punch goes completely through the wall of the drum, says Adam Christensen, applications engineer for Promess.
“At the same time, the press is monitoring the force,” he explains. “You want to see a specific force for the punching operation. If it takes too much force, it might mean the metal is too hard or the punching tool is getting dull. You don’t want a dull punch, because it could bend the drum or produce burring on the opposite side.”
Hines engineers chose a Promess press for the operation because they wanted precise control of the position and speed of the ram. “The drum is supported centrally, and there’s just enough space cut out to enable the punch to work,” says Christensen. “You want to fully penetrate the material, but not go through the nest.”
“We’ve supplied quite a few presses for Hines,” he adds. “One of Hines’ customers has six or seven of our machines, each for different size drums.”