Selecting a Gear Reducer with High Level of Input Speed

Hello everyone. Let me preface this inquiry with this statement by saying that I like simplicity and duplication whenever possible, and I like to keep things available as "off the store shelf" as much as possible. I follow a philosophy that dictates when I design something for use in a manufacturing operation that I should (as much as possible and practical) use electric motors and drive controllers that are of the same type/configuration as used on other equipment. The practice of this idea reduces the amount of items required for a working spare parts inventory. It is certainly not applicable in all situations, and yes, it is sometimes just a bit of overkill. But it has its merits when it can be applied. I desire to keep all items of North American manufacture as well, and keep exotic items to an absolute minimum.

I have an application for that I seek the best choice of equipment in regards to reliability and low cost. The application begins in a simple manner that I will describe below. For the same of simplicity I will keep all units avoirdupois.

A hollow aluminium roll with a static weight of approximately 350 pounds must be revolved within a speed range of 1.5 rpm to 550 rpm. This roll applies a slight tension of approximately 0.5 pounds/lineal inch to a moving web. The effective width of the roll is 170", hence, the basic torque required to maintain this level of tension is 27 pounds/foot. So as you can see, there is neither much rotating mass nor torque required. The most serious thing is the ability to maintain a preset (and adjustable) tension at all line speeds. This application operates 24/7/365 with a maintenance service interruption every three (3) weeks or so.

So, I have the motor and drive controller that I need - an inverter-duty 1.5 horsepower (@ 1,800 rpm) motor that is designed to operate continually at rotating speeds as high as 3,600 rpm. Also, through proper reduction this motor can develop and maintain plenty of torque to accelerate with, and match the line speed. The centerline of the motor will be vertical, and the output shaft of the gear reducer will be horizontal. The preferred type of reducer uses a combination of worm and helical gearing. The motion is always in the same direction and there is no shock. The ambient temperature is about 110 degrees Fahrenheit.

We all know that when using frequency inverters, rotating motion stability can suffer if the motor turns too slowly under light loads or when engaged to turn an item that has been stationary. So, I can choose a gear reducer whose final reduction ratio allows me to use a higher motor output speed to start the motion of the roll and keep it stable when the machine is in "crawl" mode. I would also be able to take the motor to full design speed when the machine is moving at maximum nameplate speed. The motor does not care as long as it is not overloaded. The motor is not the issue here.

In my case I need a gear reducer with a fairly high ratio because of the roll diameter. I need to develop 550 rpm at the output shaft of the gear reducer in order to achieve the maximum nameplate speed of the line - plus provide the slight overspeed that will increase the web tension. Here's the problem; I would like to accomplish this while making motor revolutions that are near the nameplate maximum (3,600 rpm) of the motor. I can use a gear reducer with a very commonly available ratio of 6.6 : 1 and this allows me much variety as to manufacturer and exact configuration.

So; what's the problem? The problem is the high input speed of the gear reducer shaft since most units designed to couple to an electric motor (in this case a 145TC frame) are designed and rated for use at 1,800 rpm. Since I am using a 1.5 horsepower (@ 1,800 rpm) motor I can simply choose to use a gear reducer that is rated for at least 3.0 horsepower at 1,800 rpm in order to satisfy the service factor idea. But there are other problems with such simple ideas!

The line speed of the gearing in at least the first stage of the reducer will be twice the rating specified by the manufacturer, hence, oil slinging (possible oil foaming), harmful vibration harmonics, and higher operating temperature will be the result of this. And we know exactly what this does to reliability. It matters not how many levels of reduction exist, as the input speed is what it is. I like simplicity and duplication whenever possible, and I like to keep things available as "off the store shelf" as much as possible.

I do not want to use exotic lubricants, recirculating oil coolers, or anything that will add cost, complexity, and more points of contention regarding reliability. Has anyone ever tried to operate a commercially-available gear reducer at twice the design input speed and been successful from a standpoint of longevity? I can of course bite a small bullet and limit the maximum motor speed to 2,000 rpm and select a reducer that has a final reduction ratio of about 3.7 : 1. That would make the applications engineers at the OEM's happy, but of course they do not pay me. It also limits the variety of gear reducers since it is a fairly low ratio. Flexibility is the idea here, but I will of course do what I need to do. Any ideas/suggestions?

Best Regards,

Ing. Robert Forbus