Nathan B. Crane and Dr. Larry L. Howell, Mechanical Engineering
A compliant mechanism is a mechanism in which some or all motion comes from deflection of its members. Several years ago, a one piece centrifugal clutch based on a compliant mechanism was developed and implemented in several products. It offers many cost and assembly advantages over traditional clutch designs containing at least five components.
Centrifugal clutches are very useful in any application in which it is desirable that motion or torque be transmitted only when the drive shaft is rotating above a critical speed. Previous tests have shown that compliant centrifugal clutches can be successfully incorporated into weed trimmers and model helicopters. However, their application to other products is limited because a proven design method is not available. The focus of this project was to design, build, and use test equipment to validate performance prediction formulas.
The test equipment must measure transmitted torque and rotational speed. The equipment would accelerate the clutch, allow speed adjustment and be adaptable to a wide variety of clutch designs. A candidate design model of clutch performance was developed and refined to aid in the identification of the necessary speed, power, and torque capabilities of the components. Finite element analysis was used to verify arm deflection, contact location, and transmitted torque predicted by the design model. This analysis helped identify errors without the expense and difficulty of measuring parameters such as arm deflection and contact location.
An available numerically controlled lathe was selected as the power source. The lathe provides power source, speed adjustment, speed measurement, alignment, and reliable mounting configurations. Other comparable options would have cost thousands of dollars. Other components include a clutch mount, clutch drum, and instrumentation. A clutch mount was designed to fit into a standard lathe chuck with four holes on a one inch bolt hole circle for mounting the clutch. The drum was designed with an inner diameter of 2.015 in and a depth of .3 in. These sizes were chosen to accommodate the size capacities of the prototyping tools and the 1/4″ thick sheet polypropylene stock generally used. The test equipment can be used with one of two instrumentation options. The simplest requires attaching a static torque sensor/force gauge to the clutch drum and capturing the peak torque/force applied when the clutch is accelerated to a constant speed and quickly decelerated before the clutch heats excessively. This method rapidly loses accuracy at higher speeds since the energy dissipation is proportional to the cube of the rotational speed.
The other instrumentation option is to dynamically measure the speed of the clutch drum and the speed of the clutch using tachometers. The drum acceleration versus clutch speed can be calculated from the experimental data. The applied torque can then be calculated from the relationship T = I* where T is the applied torque, I is the moment of inertia of the clutch drum, and is the angular
acceleration. The method may be sensitive to signal noise because it is dependent on the numerical derivative of the velocity measurements. However, the advantage of this method is that the entire torque versus speed relationship can be measured in a single test.
A test clutch and drum were machined out of polypropylene. The first test method was applied to the prototype clutch. Figure 1 compares the predicted torque from the model to the output torque measured with this method. The agreement between predicted and measured values at low rotational speeds support the developed design model. It is likely that the error at higher speeds is due to rapid heating of the polypropylene and the resulting changes in material properties. Further testing should be done to evaluate the repeatability of the torque measurements.
In conclusion, a simple but effective test method has been devised, built, and tested. Preliminary test data supports a proposed design model of the clutch performance. However, the test methods need to be refined. Repeatability can be improved by reducing clearance in the clutch drum mount. The heating problems in the clutch could be reduced by machining new components from aluminum.