Greetings from the "Frozen Eastern Wilderness,"

I am unsure how to preface my answer as I do not know what type of blower you are using or its lubrication type and this restricts my thinking about your bearing failure.

Centrifugal blowers operate in one direction only due to air inlet configuration etc.

Rotary lobe blowers can be operated in either direction for either vacuum or low pressure high

volume air service.

Bearings of any type can run in either direction,

bearings will only run so long with out lubricaction.

If your blower is running in reverse without lubrication assuming you use pressurised oil the bearings are running in reverse with only a film of oil on the bearing parts.

I am assuming you are using the either cup and cone configuration or roller bearings. Cup and cone bearings are for low speed and high speed aplications, where ball, barrel, tapered barrel bearing types are for slower speed high load applications. It all depends on the low bidder for the application.

All my experience stems from mining service for rotary lobe blowers and high RPM centrifugal blowers for aeration in sewage treatment.

Rotary lobe blowers are usually run through a engine power take off or through a pullley and belt system running off an electric motor at a reduced rpm delivered through the electric motor and pulleys mounted on the electric motor shaft.

the blower lobe set is driven by a pair of spur gears one driving and one driven.

The gears are splash lubricated by gear oil in a small reservoir where the gears rest. The bearing

sets on the blower are ususally greased and are unsealed bearings as a rule.

The systems used in diesel powered roof drills require 2 sets of vacumm systems requiring separate check valves for the electric and diesel operation.

This brings me to the point where we discuss a rotarty lobe blower running in reverse:

Since the above operating systems are dual mode for diesel and electric operation the check valves

to direct vacuum in the system will occasionaly

have drilling fines collect on the swing check valves and stick open and not seal resulting in low or no vacuum for drilling.

If you do anything you should install a check valve or flap door to gaurantee one direction and one direction only for air flow this will also stop the blower running with out lubrication in reverse. please tell me more: power used for driving blower, blower type,speed, lubrication type etc.

lzaharis ■

Ross

Have you had the bearing manufaturer do a failure analysis on the failed bearings? This analysis may lead you in the right direction.

Gary ■

Ross,

You can rule out the vacuum blower (centrifugal type) from causing this wear. These blowers are not the constant displacement type, their flow reduces with pressure they have to pull. When the main blower is not running, the only source of air inlet into the conveying system are the very narrow clearances between the rotors and the housing. The centrifugal blower may be able to pull some air through these clearances but it will soon reach its pressure limit.

You should also rule out slow speed. Bearing life, in general, should not reduce but increase with rotor speed unless the speed is extremely low.

One possibility you may like to check is the discharge pressure setpoint of the blower Vs the normal operating pressure. If it is set too close, blower will start, stop, and restart too often. Too fequents start-up loads can cause wear of the bearings because the start-up loads can be 3 times the normal loads.

Regards,

Amrit T. Agarwal

Consulting Engineer

Pneumatic Conveying Consulting LLC

Emailolypcc@aol.com

Ph and Fax: 304 346 5125 ■

Ross,

who is it that maufactured your blower and what is

it that you produce or convey to utilise it-something is still missing as I have never heard of a screw type blower other than a screw compressor used in compressed air service- I knew at one time that "Gardner Denver" built a screw/lobe blower over 20 years ago but they no longer exist as far as I know.

Amrit is right sbout the bearing life-I forgot to mention it last night-tired and past my nap time :^(-

Signed,

Still stumped here in the "Frozen Eastern Wilderness" ■

Hi Ross

Have U found out the real root cause of this issue for U?

U should take the brgs down to FAG or SKF for their assessment on the failed bearings.

The only thing that I can think of is that your blower has the suction and discharge at opposite ends of the shaft ie screw type and that one of the bearings requires axial loading (which it will get when running under load). It it is freewheeling, then this brg may not be getting the axial loading and the rolling elements may be skidding...this will lead to early failures.

I am only guessing as I do not know the condifugration and design or your blowers. FAG or SKS should be able to help U further, they will need a cross sectional view of your blower.

If the freewheeling is really the issue, U could install a spring loaded non return valve in the line...so that the suction will not lift the valve off and the blower will not freewheel.

Cheers

James ■

Ross,

Rotary lobed blowers are usually splash lubricated by oil sumps at either end. As such when operating below minimum recommended speed the bearings tend to fail due to insufficient lubrication and heat dissipation. Also due to a low volume of air flowing through the machine cooling is inadequate.

However in your case the blowers are not operating against a differential pressure ( 3-4" H20 is negligible for a roots blower )

and should not cause such frequent failures.

In order to check this what I would suggest is for a very small hole to be drilled in the pipe downstream of the blower. This will equalize pressures in the pipe and prevent any vacuum from being set up in the pipework.

If the hole is sufficiently tiny there should not be much of a air loss when the blower is running.

If the blower is being used for pneumatic conveying purposes ensure that the hole is prior to the product pick up point.

Hope this helps.

Regards

Harish Krishnan ■

Harish,

I totally forgot about that-it is equal to drilling a small hole in a sump pump discharge line above the check valve to reduce pump loading at start up.

lzaharis ■

Hello Ross,

I understand the application exactly. I've never heard of such nonsense. This same blower manufacturer will advise you to periodically turn the rotors by hand during long term storage in order to avoid future bearing failures. Why would they now tell you that you cannot slowing rotate the machine during idle times. If there is no differential pressure, and no heat being generated, then the only stress being placed on the bearings is the weight of the rotors themselves. I'm not a bearing expert, but I doubt that any ball bearing manufacturer would say that slow speeds could cause premature failure.

I think you need to concentrate on what type stress the machine is under during operation, not during idle times. There must be something not exactly right (vibration, heat, pressure cycles, pulsations, etc...). Keep looking.

Happy New Year,

Bob ■

Bob thanks for that, I knew that the manufacturer was was just making excuses. ■

I would also like to endorse Bob Reischl's reply. Until then the possibility of surge had not been mentioned adequately.

Ask the blower people why the motor bearings aren't packing up as well! ■

ross,bob,and john I forgot about the bearing type as a rule they are ball bearings in grease lubrication, I wonder if the ball bearing in question doe not have a full compliment of balls in the bearing raceway?, but the problem still does not make sense from a bearing life or application point of view still, simply because all the roots blowers I have assembled have never had a bearing set without a complete ball assembly.

I also forgot to add that maybe just maybe the grease is not heavy enough? But the bearing cases should not leak either.

One more possibility is the gaskets in the blower housing -never having seen a screw type blower I am assuming they use sealing gaskets for the air ends.

Still scratching my head here in the "Frozen Eastermn Wilderness"

leonz ■