ELECTRIC MOTOR COMPANY DESIGNS LIGHTER, SAFER TREADMILL COMPONENT USING FEA SOFTWARE

The 42 Frame DC Motor is used primarily in treadmills. A belt mechanism attached to the motor's shaft moves the treadmill's walking surface. If the motor's endshield cover experiences deflection over twelve one-thousandths of an inch during operation, the endshield cover distortion will shift the internal rotor assembly and force it to contact other motor components, which could stop the running motor.

October 23, 1998, Pittsburgh, Pennsylvania -- During peak exercise times at a fitness facility, a treadmill may run nonstop for several hours. The electric motor that powers the running belt must endure daily usage by people of various sizes and fitness levels. Leeson Electric Corporation designed a lighter, safer endshield cover for treadmill motors and ensured that it could withstand at least 40,000 operational hours using finite element analysis (FEA) software by Algor, Inc.

Leeson Electric Corporation is a leading manufacturer of electric motors, gear motors and gear boxes. Leeson redesigned its 42 Frame DC Motor's endshield cover to reduce the part's weight and minimize deflection during its manufacturing and application processes using Algor's linear static stress analysis software. The company then used Accupak/VE Mechanical Event Simulation for Virtual Prototyping with Linear and Nonlinear Stress Analysis software to verify the linear analysis results and determine the ranges of stress and deflection in the endshield cover. Deflection of three ten-thousandths of an inch can mean the difference between a quiet, long-running motor and premature failure. Analyzing the endshield cover's deflection on a computer saved the company time and money by reducing the number of necessary physical prototype tests.

New DC Motor Endshield Cover Appeals to Larger Market

The aluminum 42 Frame DC Motor endshield cover is 4.8 inches in diameter. The two endshield covers on either side of the motor protect the moving motor parts from the environment and allow easy access to the motor's brush for service or replacement. The brush is a component that connects an electrical source to the rotor assembly. The DC Motor is used primarily in treadmills and also in lift equipment for disabled persons, gear boxes, refrigeration systems and pumps.

The redesigned endshield cover is lighter, reducing manufacturing costs, and meets the safety standard IP21 established by the International Electrotechnical Commission (IEC) in Europe. The IEC's IP21 standard requires that endshield cover openings be less than 12mm in diameter to prevent fingers from entering and contacting rotating or electric current-carrying motor parts. In the United States, larger openings are acceptable if a motor operates in an enclosed area where contact with human fingers or other objects is unlikely. Meeting the IEC's requirement allows Leeson to sell the motor to European manufacturers that must comply with IEC standards.

The front and rear endshield covers protect the motor's rotating components from the environment and fingers from injury.

Leeson needed to test the new endshield cover to ensure that it experienced minimal deflection during its machining process when it is rotated and shaped on a lathe and during its application as the motor operates within a moving treadmill. Small deflections in the endshield cover can cause internal motor components to rub against each other, leading to wear and premature failure.

Linear Stress Analysis Used to Test and Optimize New Endshield Cover Design

Aleko Sotiriades, a mechanical engineer with Leeson's Research and Development Department, first conducted linear stress analyses of four different rear endshield cover design models to determine which one experienced the least amount of stress and deflection under machining and application loads. He did not analyze the similar front endshield cover because its outer wall that connects to the motor is shorter than the rear endshield cover's outer wall. The front endshield cover's outer wall is therefore not subject to cantilever loads that cause out-of-plane bending stress, making it inherently stronger than its rear counterpart.

Sotiriades worked with the 3-D solid models from SolidWorks in Algor and used Algor's Merlin Meshing Technology to create a surface mesh. Then he refined the surface mesh in critical areas, such as the bearing bore at the endshield cover's center and areas where the ribs meet. He used Hexagen, Algor's automatic 3-D solid brick element mesh engine, to create a solid brick mesh.

He applied the same two sets of loading conditions to all four models. The first load represented forces during the endshield cover's machining process when it is rotated and shaped on a lathe. Four squeezing clamps on the endshield cover's perimeter hold the part in place on a lathe. If machine operators affix the endshield cover too tightly to the lathe, the clamps can cause critical deflection in the bearing bore. The bearing bore surrounds the shaft hole on the endshield cover's interior. It houses the bearing that allows free rotation of the motor's flywheel. When the endshield cover is removed from the lathe, residual deflection must be minimal. If the bearing bore deflects over three ten-thousandths of an inch, the bearing inside will misalign with the shaft, leading to excessive motor noise and premature bearing failure.

Sotiriades then applied loads to all four models that represented forces during the endshield cover's application. These included radial forces from belt tension and the motor's rotating flywheel and axial forces from two bolts that attach the endshield cover to the motor. The bolts squeeze the parts together and can cause extreme force if turned too tightly by assembly or maintenance personnel. If the combined load parameters cause radial deflections greater than twelve one-thousandths of an inch, the endshield cover distortion will shift the internal rotor assembly and force it to contact other motor components, which could stop the running motor. Leeson designs the endshield cover to withstand four one-thousandths of an inch to ensure infinite life, guaranteeing its function beyond the average bearing's 40,000-hour operating life.

Sotiriades optimized the design that experienced the least amount of stress and deflection and had the lowest mass. He removed excess weight from the endshield cover's outer wall, still meeting the minimal deflection requirements. He was able to reduce the endshield cover's weight by nearly 50 percent. He then verified the linear results with nonlinear stress analysis software. Nonlinear analysis uncovered more detailed information about the model's range of stress and deflection.

"I used linear analysis as a quick, filtering technique to find the best working shape for the endshield cover," said Sotiriades. "I then turned to Algor's Accupak/VE Mechanical Event Simulation software to verify the linear analysis results and provide an enhanced profile of the endshield cover's stress and deflection. The presence of minute deflection could be devastating to the motor, so I wanted to be absolutely sure about the linear analysis results."

Nonlinear Analysis Verifies Best Endshield Cover Design

To verify the linear analysis results, Sotiriades used Algor's Accupak/VE Mechanical Event Simulation for Virtual Prototyping with Linear and Nonlinear Stress Analysis. He first modeled one-half of the rear endshield cover to save analysis time testing it under machining loads. The complete model consisted of nearly 10,000 elements and the loads representing the four squeezing clamps were symmetrical. He then analyzed a complete model under application loads.

Sotiriades analyzed the models using the von Mises strain hardening and von Mises kinematic hardening stress criteria to determine the minimum and maximum ranges of stress and deflection. He found that stress did not exceed the material's endurance limit and deflection values did not exceed those allowable during the endshield cover's machining and application processes. He used Algor's post-analysis visualization capabilities to view the analysis results in detail.

The nonlinear Algor model of the endshield cover under loading conditions during the motor's application, viewed from the inside. The endshield cover experiences radial forces from belt tension and the motor's rotating flywheel and axial forces from two bolts that attach the endshield cover to the motor. The linear and nonlinear analyses found that the combined load parameters did not cause greater than the allowable four one-thousandths of an inch of deflection.

"I appreciate Algor's post-processing capabilities. I can manipulate my model view to study the analysis results from different perspectives. And it is easy to create graphics of the model that I can use for presentations," said Sotiriades.

Keeping Those Motors Running

Leeson manufactured 22 aluminum endshield cover prototypes to verify that new deflection-causing forces would not arise during the endshield cover's production and assembly as a result of its new geometry. The prototypes also confirmed Algor software's analysis results.

To verify distortion during the machining process, the prototypes were measured with a visual scanning device, placed on a lathe and measured again when removed from the lathe to detect deflection. Some of the prototypes were then tested in actual customer applications under normal loading conditions for over 10,000 hours, the minimum life of a treadmill. Leeson tested the remaining prototypes in its laboratory. To save 7,500 hours of test time, Leeson applied over twice the normal loading conditions and ran the motor for 2,500 hours.

No prototype failures were reported. The company has sold 22,500 motors with the new endshield cover since it was introduced in May 1998.

"Virtual prototyping with Algor's Accupak/VE software shortens the length of the design cycle by reducing the number of required physical prototype tests," said Sotiriades. "Leeson Electric Corporation saved time and money optimizing one rear endshield cover design with finite element analysis software. We only had to verify the results of the final Algor model of the rear cover with laboratory tests."