Selecting The Right Shaft Coupling

When designing machinery, how do you begin to select the right coupling for a drivetrain application? How do you find one that will not only compensate for misalignments in the joined shafts but will also provide the flexibility, torque capacity, corrosion-resistance and other performance capabilities that the application requires? Here’s a case in point that provides some of the answers.

Tri-Turn Technologies, Inc. (Cleveland, Ohio), manufactures the Tri-Turn 383, a true-CNC, three-spindle, screw machine that was designed to fill the gap between one- and two-spindle CNC lathes and multi-spindle screw machines. The Tri-Turn 383 has three, main, 29-hp, 5,000-rpm spindles with 1.5-inch-diameter bar capacity. The bar can be fed out from the main spindle as machining takes place, simulating the operation of a Swiss screw machine; machining can be kept close to the spindle to minimize deflection and maintain optimum machining results.

Each of the machine’s three spindles is opposed by a counter-spindle that allows the part to be transferred back and forth as needed to machine both ends of the part. The counter-spindle can also serve as a tailstock for long parts, act as a live-tool spindle with high (7.4) horsepower and permit a doubling of the live tool rpm by counter-rotation.

“In our prototype design, we used two steel collars and an intermediate member to connect the drive with the counter-spindle assembly,” explains Ken Sommers, engineering manager for Tri-Turn Technologies. “Aligning these components was difficult and put extra stress on the motor bearings. The connection also was too rigid resulting in vibration, especially at higher speeds. There was no way of compensating for misalignment with the connection. Also, the vibration caused by the rigidity of the connection generated heat around the motors that could compromise the system’s operation. So we looked for a better way to connect the drive and spindle assembly.”

Tri-Turn engineers considered a variety of coupling alternatives. They decided on a Zero-Max CD (Composite Disc) Model 6A45C coupling manufactured by Zero-Max, Inc. (Plymouth, Minnesota). The CD coupling is a hybrid coupling that combines the best features of steel disc and elastomeric couplings. The heart of the coupling is an “open-arm” disc made of composite material (photo). The design of the coupling enables it to provide the high misalignment capacity found in many elastomeric couplings, but with higher torsional stiffness. In most applications, the CD coupling allows for a 0.4 mm parallel misalignment and angular misalignment of up to 3 degrees.

According to Zero-Max, CD couplings are superior to steel disc couplings for damping and isolation of shock and vibrating loads. They reduce or eliminate fretting corrosion and greatly reduce stress fractures at bolt hole locations. They also provide excellent chemical and moisture resistance in hostile environments that prove difficult or impossible for elastomeric or steel disc couplings.

Tri-Turn selected the CD coupling for its ability to handle misalignments, its ability to dampen vibration and its overall durability. Also, the construction of the coupling enabled it to handle the machine’s quick acceleration/deceleration and the torque loads imposed by the connected components.

“The improvement with the CD coupling was evident as soon as we installed it,” Mr. Sommers recalls. “With its clamp style hubs, installation was just a matter of inserting the coupling and tightening up both ends. No time-consuming alignment of the connected elements with special tools such as lasers was needed. Upon startup, there was no vibration at all. The system ran smoother and much quieter than the original design and without the excessive heat build-up we experienced before.”

The coupling is a standard model with clamp-style hubs (photo). It is 4 ½ inches in diameter and has a 1.62-inch-diameter bore with a keyway. It has high torsional stiffness and handles speeds to 5,600 rpm.

Zero-Max offers over 40 standard models and sizes of CD couplings that meet the needs of most applications. The company also designs and manufactures custom CD couplings, adding to or reducing coupling flexibility, increasing strength and stiffness and making similar changes as the application requires.