I have a PPG. hog turning 900 rpm that I have been monitoring since we installed it. The hog is driven by a 300 hp 1185 rpm motor coupled via belt and pulley configuration. The problem that I am seeing is that the motor has increased in vibration over the last couple of months. After going through the data I am seeing that the imbalance is actually coming in at the hog speed at 9.1 mils displacement on the motor and the peak list for the motor is 100% sub. which it should be. The thing that has me a little confused is that the motor is showing 9.1 mils displacement at hog freq. but the highest displacement measured on the hog is 3.4 mils displacement. The motor is 194% higher than the hog which is actually what is out of balance. Does this make any sense????

I have often seen something like this when there is a problem with belt tension. It most often happens here when there is inadequate structural support for the motor.

What is the motor attached to?

Is it possible that there is a little bend in the hog shaft, causing it to pull on the belts?

My guess would be sheave misalignment or hog sheave eccentric. (what's a hog?)

These put a force on both machines but it may be that the motor has lower dynamic stiffness at this frequency.

This is very common on belt driven machines especially "hogs" if the motor mounting is "weak".

There may even some resonance in the motor mounting (frame or whatever it is sitting on).

Hogs are notorious for causing this type of problem. They are usually much heavier and more massive than the motor and can cause the frame work to be at "its mercy" to do with almost whatever its wants, especially if mounted on a 4 or 6 legged platform off the solid ground.
They also get out of balance quite frequently.

I would check the motor and hog frame and if all is ok I would check the sheave alignment and also balance out the 3 plus mils of the hog if it is still there after the sheave alignment and see what happens to the motor.

Bumping around on the frame and motor with the hog down might show you a little something also.

The motor and hog rotor are mounted to a skid like framework from the factory. As far as I know the sheaves have never been taken off or moved but I will be checking them on our next down day with a dial indicator to make sure that everything is true and square to both shafts and that the motor and hog are square to each other.

I'm with e-pete WHAT'S A HOG?????

HOG = Large motorcycle ?

quote:

I'm with e-pete WHAT'S A HOG?????

A full grown pig?

Naw! Seriously look here and you will see one type.

http://www.jeffreycorp.com/size_reduction/Wood_Hogs/EZ_Access.cfm

Would it be a tasty animal that say oink, oink? :-) I also vote for eccentric sheaves possibly mostly in the Hog end if that freq. is seen in motor end. Poor animal. Olov

Ralph,

Thanks for saving me the typing that you did so well.

Walt

OK I guess I left myself wide open for the comments That's why I like this board so much, MOST everybody has a sense of humor.

Thanks for clearing that up Ralph. When I started this post I never thought about the tastey kind of hog you cook for 20 hours and drink beer while its cooking. (can't wait for summer)

Is the hog a swing hammer style or one with inserted teeth? We got rid of our swing hammer unit for one with teeth and the trouble started. The rotor weighs in at 18,000 lbs and as mentioned, shakes everything else. It is direct drive on a second floor of a tower. Problems included natural frequency of floor equal to running speed and loose discs on the rotor. Finally, after adding several braces, increasing floor stiffness, and pulling the hog out, disassembling it, and fixing shaft fits, it now runs acceptable. Only, it is high maintenance.

Was it made in Jacksonville, FL?

Gary B

It's very common on most anything that is belt driven for the motor to vibrate excessively at the driven unit speed. I see this a lot on fans, especially if the motor sits on a frame attached directly to the fan pedestal. Quite often it is too much belt tension.

I would just back off on the tension a little. If the belt design is adequate (horsepower-wise) and the sheaves and belts are in good shape, then you can run with the belts fairly loose. A v-belt doesn't actually depend on belt tension for it's holding power... "grip" is a function of the belt deformation (the sides pooch out) producing friction against the sheave. If a belt doesn't slip, it's tight enough. Belts "flopping" on the slack side is seldom a real problem.