I was asked the other day by a customer to quote on a gear motor to drive a slow moving conveyor. No problem, we do it all the time. “How much horsepower do you need and at what speed?” I asked. “Oh, I’m thinking 1HP at 1RPM” he replied. After some more questions I discovered the conveyor was quite short and only lightly loaded. He just needed a slow speed. So I asked him “why so much horsepower?”, and he told me he wanted to error on the safe side. After some quick calculations I showed him that his torque requirements were actually very, very small and that he would be wasting money on buying a much larger gear reducer and motor than was necessary.
This is a very common occurrence, customers not taking the time to work through the calculations to see what their actual torque demand is. Many machine designers have a “bigger is better” attitude, some don’t know how to work out the torque, others don’t think it’s important.
So what’s the big deal? There are a number of problems with oversizing a motor and gear reducer. As a manufacturer of a machine a main goal should be to produce a lean design to make the overall cost of the device as competitive as possible. There needs to be a happy balance between the safety factors used for deriving motor power and also the mechanical safety factors in the gear reducer selection. There is no point in selecting a gear motor that will last 20 years, on a machine that is designed with a 10 year life cycle. Oversizing the gear motor defeats the competitiveness goal, potentially causing lost sales of the machine.
Using a larger motor will also mean larger rotor inertia. If the gear motor is going to be cycled on and off in any sort of rapid fashion then this extra inertia will need to be accelerated and decelerated without any positive effect. In cycling duties it is best to have as small a rotor inertia as possible.
The purchaser of the machine will also bear the burden of higher running costs than is necessary. With today’s high energy costs this could add up, not to mention the environmental impact. As an OEM supplier of gear motors for 15 years, VL Motion Systems has developed a vast library of application experience from simple conveyors to complex systems requiring precise torque characteristics. We are very willing to help machine builders and designers with their calculations either to provide assurance or confirmation that their torque estimates are accurate and that the most economical and efficient gear motor solution is used. Do you have an application we can help you with?