While that depends upon the application the gear will be used in, it’s just as important to understand the differences between the processes. When rolling a gear, the material is displaced so it’s critical to start with material that’s ground to a certain size or diameter. There’s so much pressure involved in rolling gears that if the diameter is oversized, for example, the die will start overfilling, resulting in a distorted or inferior gear.
Problems can also develop if a manufacturer starts with an undersized diameter. More than likely, you will end up with an undersized gear with potentially functional problems. Keep in mind that gears are produced to mate with other gears. If not manufactured correctly, then performance issues will arise with whatever application the gear is being used for.
Rolled gears are basically formed out of material. Think of any gear as a set of teeth. The rolls press down on the material in spots, moving the material into the high spots – called the peaks – or low spots – called the valleys - of the gear’s teeth. The starting diameter is close to the pitch diameter, which is the spot halfway between the valleys or peaks.
However, cutting a gear is a little bit like sculpting. Also referred to as hobbing a gear, manufacturers are actually cutting away material. They start with the diameter of the largest tooth or area on that gear. Then they cut away and remove material to make the smaller teeth or valleys between teeth.
Pros and Cons
There are several factors to consider when deciding which process to use.
- Because the process involves compressing and moving materials, these gears are very strong
- The cycle time per gear is roughly half that of cut gears
- They’re less expensive, generally half the price as cut gears
- They’re more precise. The American Gear Manufacture’s Association (AGMA) sets the standards or precision between teeth for gears, which ranges between one and 14. When rolling gears, on average, you can only reach five, maybe six. If you need to go above that, then your gears will need to be cut.
- The cycle time per gear is approximately twice that of rolled gears
- They’re generally less noisy than rolled gears, producing less vibration or friction
- Many times, cut gears produce less heat and offer a better fit with other mated gears.
There are plenty of applications for both types of processes. For example, rolled gears are generally used to manufacture agitator shafts in standard washing machines. Millions are produced each year. But cutting the gear would be overkill, negatively impacting your bottom line. You would end up paying more than you need to, especially when the rolled gear has been proven to perform well.
Besides agitator shafts, other applications for rolled gears include parts for lawn mowers, trash compactor and also golf carts.
Some applications for cut gears include windmill generators, transaxle parts for rear-wheel drive vehicles and also kitchen stand mixers. People don’t want to hear a lot of noise when driving a car or mixing cake batter in their kitchen. But noise level isn’t a concern when cutting the grass or tilling a garden.
Sometimes, customers try to cut corners, which doesn’t always work in the gear industry. Don’t assume that cut gears, which are more expensive than rolled gears, will always cost you more money in the end.
Manufacturers that are customer service oriented often educate customers about both processes. Consider one customer who cut one gear but then used powdered metal as a mating gear in an attempt to cut costs. The powdered metal gear could wear down quicker, causing failure and customer returns. It could be less costly overall to use two cut gears, or even two rolled gears that mate very well together.
Depending upon the application, manufacturers can sometime offer alternative approaches. In this scenario, the most effective solution would have been to roll both gears, which would have produced a better fit and saved money.
When deciding whether to roll or cut, you need to consider the appliance’s speed, load of the motor, heat and vibration. The general rule of thumb to follow is if the application is going to spin fast and produce lots of heat and vibration, then the gears needs to be cut. Likewise, if the appliance is going to run slow, generating less heat and vibration, then the gear can be rolled.
Ultimately, it’s your decision whether to roll or cut gears, but manufacturers can offer suggestions to help you make better, informed decisions to avoid problems down the road. Although cost is important, remember that other factors need to be considered. Work with manufacturers that have experience using both types of processes so they can lead you in the right direction.
Larry Sutter is the sales tech at Ashley Ward, Inc., a machine shop based in Mason, Ohio. With additional offices in Elkhart, Ind., Michigan City, Ind., and Green Cove Springs, Fla., the company’s nearly 200 employees specialize in manufacturing machine products and gears for the automobile, appliance, lawn and garden, defense, cable connector and hydraulic valve industries.
Contact Larry Sutter at firstname.lastname@example.org or 513-398-1414. Visit us online at Ashley Ward, Inc.
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