Gearbox rattle is a sound that is excited by the driving source such as the electric motor.
The asymmetric gear noise spectrum arises from amplitude and frequency modulation of gear-mesh excitation produced by low-frequency manufacturing and assembly errors. Gearbox and electric motor sounds from gear mesh, bearing rotation, lubricant movement, motor vibrations and interaction of the housing all contribute to the overall sound generated.
It’s impossible to eliminate all gearbox noise, because it’s impossible to cut perfect gears. Even if one could, it’s impossible to limit the effect of system dynamics. One can only minimize and control gearbox noise to the extent that it won’t be considered problematic or audible.
To solve gearbox & electric motor noise problems, the first step is to determine the type of noise that is objectionable. What’s considered gear noise depends on the speed of operation. Use qualitative and quantitative terms to describe how design factors and manufacturing mistakes play into the noise equation. Then discuss with the design team (and potential suppliers) the problems of gear noise, dynamics, measurement, and modeling.
Gear noise is generated by tooth-to-tooth load transfer that causes pressure-pulse trains radiating through the gearset and motor housing. The noise’s frequency is the product of gear rotational speed and the number of gear teeth. Most types of gear noise occur at tooth mesh frequency or harmonics in the audible range.
Gearbox noise can be very annoying — even when it isn’t the most significant noise source. That’s because it occurs as pure tones that the human ear can detect even at 10 dB lower than the overall noise level.
To minimize noise, all gearbox & electric motor components should be optimally tuned to each other. Three types of gear tooth transmission errors are spacing errors, random errors, and elastic deformations, which combine with mean profile deviations. Gear resonance can be reduced by identifying excitation mechanisms; using finite element analysis to determine the natural frequencies of individual gears; eliminating torsional modes from operating range; detuning and damping helical and bevel gears; and identifying remaining resonant problems with other elements of the gearmotor such as the housing and electric motor.
Through a series of blog posts, we’ll be exploring aspects of gearmotors regarding specific problems and applications. Subscribe to our posts to get the latest information about gearmotor and electric motor selection and design.
Quiet Gear Motors
1. Why gear motor noise isn’t just a gear problem — it’s a system problem. Motion control in churches, libraries, auditoriums and theatres need quiet gearmotors for curtains, podiums and stages that need to rotate, lift or slide, unnoticed by the audience.
2. Electric motor and gearbox noise and vibration diagnosis, analysis and design reduction techniques.
3. To solve gear-noise design problems, the first step is to determine the type of noise that is objectionable.
4. Minimize gear noise in high-speed stages by fine machining and grinding and optimized gear geometry to minimize the impact of individual gear engagement impulses.
5. To minimize gear motor noise, all components should be optimally tuned to each other.
6. Reduce natural resonance by optimizing gear motor housing designs with ribbing and non-symmetrical components — especially on parallel-shaft gearbox types.
7. Excitation from the motor’s electromagnetic field can transfer to the rotor, so look for motor rotors that are sturdy and robust.
Extreme Ambient Conditions
1. Designing your gear motor for decades of exposure to extreme ambient conditions with low maintenance
2. How to design a gear motor for extreme temperature swings such as when a forklift moves from a -30˚ C freezer to a +40° C dock door
3. Using gear motor heater jackets, motor heating bands or powered windings to prevent moisture build-up in high humidity environments
4. Typically gear motor rated installation altitude is a maximum of 1,000 m above sea level, for higher altitudes, you should consider derating the motor
5. Why is it critical to carefully determine what gear motor duty cycle and what peaks/shocks per a specific time period are experienced
High Efficiency Electric Motors
1. To effectively design high-efficiency electric motors, we must define “efficiency”. The construction materials, mechanical and electrical design dictate its final efficiency. When you consider that electric motor systems account for about 60% of global industrial electricity use, the potential savings become clear.
2. In order to make a motor more efficient we have to reduce losses in the motor. Optimize dimensions of rotor and stator laminations to provide the maximum in iron content. Quality of aluminum die casting of rotor package (use of high grade/pure aluminum). Materials/quality of steel, dimensions, impregnating medium for high-efficiency electric motor stator and rotor laminations. Use best – most penetrating – resin impregnation process – bath versus drip for optimum thermal management.
3. Electric motor stators cause of 60% of losses so in order to reduce these losses mass of stator winding must be kept larger as this increase in mass will reduce electrical resistance. Motors that are highly efficient contains 25% extra copper as compare to motors that are designed for standard efficiency models. The use of high quality of copper wire that meet standards per DIN EN 13601, UL Norm with proper number of insulation coats is important to high-efficiency electric motor design
4. Use high quality balancing machine; balance at motor output speeds application will run; has limited effect on efficiency but impacts operating noise and life expectancy that is also important for maximum use of resources
5. Tighten manufacturing tolerances to allow optimum air gap between rotor and stator. Selection of best suited, low friction bearings suited for output speeds. Assure tight bond between stator and motor housing to provide best cooling performance
6. High-efficiency permanent magnet electric motors. Type and quality of magnets (rare earth versus iron magnets etc.). Select inverter that can provide sensorless operation. Programming and optimization of inverter