Roots Blower Problem

Dear Colleagues

Two roots blowers, same model and make and addressing the same application. One of them is having problem as described below

1. Pl see the attached spectrums.
2. Frequent bearing failure at location no. 3.
3. High acceleration and enveloping (gE) readings at bearing no. 3,4, & 5. Overall vibration levels in the range of 7-9 mm/sec on all the blower bearings.
4. Similar frequency components in all velocity, acceleration & enveloping spectrum as shown for the bearing no. 3 in the attachment i.e. dominant frequency of 2 X blower RPM.
5. High temperature of bearing no. 3 & 5 i.e. approx 105 Deg C as compared to the another blower which has a temp of approx 78 Deg G.
6. I understand that normally, this type of blowers will have typically vibration levels in range of 6-8 mm/sec. However, I am not sure of the G levels which can be typically allowed on this type of machines.

You expert comments on what could be the possible cause. Pl let me know if any further information required.

Regards VRG


Original Post

What are shaft speeds: Motor, lobe-1, lobe-2? What is motor horsepower? How many lobes on blower rotors? Does the motor have a VFD control?
The #3 bearing is sensitive to load from belt drive. Do you have accurate sheave alignment, offset and angle, in both planes? Do you have correct belt tension. Are belt vibrations on both slack and tension spans low? Do you have correct bearing lube oil and oil level? Do you have any shaft seal leakage? Have you measured blower load and compared to motor and blower ratings? Are the timing gears on the drive or non-drive end?

Dear Walt

Motor Speed - 1480 Rpm. No VFD.
Blower Speed - 2345 Rpm
Motor HP - Will let you know.
Sheve Alignment checked and found OK.
Belt tension has been checked and is kept same as the other running blower.
Timing gears on the NDE side.
Oil Levels are OK.
Rearding the blower load, shall come back to you.
My experience with these blowers is the velocity and acceleration levels vary so much depending on load, speed and operation that it is very difficult to comdem one without trended data. I have seen the velocity readings range on good blowers from 0.1 IPS to 1.2 IPS and the g's vary from 2 to 48 on good machines running for years. Walt has a good point. The #3 bearing is going to be susceptible to belt load. Any time I see the DE bearing on a belt driven machine fail frequency, I recommend loosening the belts even below recommended tension.

John J
Are the motor and blowers mounted on frame that isolates it from the floor?

We set vibration limits a with low alarm of .8 ips and high alarm of .95 ips. Can't remember the g's alarm but most information suggests monitoring the velocity. Roots blowers are heavy built and tend to have metal to metal contact routinely. Lobe clearances are tight.

Our plant had some roots blowers and the drive bearing on one unit seized up and broke the shaft. The next install was checked and the sheave was found mounted out on the end of the blower shaft. This apparently caused excess heating. The sheave was moved closer to the blower and belt alignment adjusted. Temperature decreased and ran for along time.

Might check the sheave location.

Things that can cause problems (other than the other good suggestions) are excessive rotor contact, air filter plugging up, if to much dust is in blower lobes (air filter failure) that can impact the lobes which will effect drive bearing. If timing gears are off you probably would not get much run time at all.

The mounting location of the motor to the blower is important. The manual should have recommendations as to mounting locations for sheave alignment.

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