How to Design an Airtight Tubing with a Motor Shaft?

[Mormont]

Hello All,

I am new to designing, and I need to design an air tight tubing with a motor shaft running thru. Following are representative pics:

https://www.physicsforums.com/attachments/215901

https://www.physicsforums.com/attachments/215902

Blue arrows indicate air flow direction.
Red part is a shaft connected to a motor with top speed of 1500 RPM (mostly oscillatory motion)
Yellow part is either a Sleeve Bearing (Bushing) or a Ball Bearing.

Queries:
1. Can a bearing/bushing be used inside a split housing as shown. If yes, do I need add a radial gasket for positive contact?
2. What would be the tolerance/gap between the bushing/bearing and shaft for low friction rotation and leak proofing.
3. Is a sleeve bushing right option for this design?

Any design improvement can help.

Thanks in advance

 

[Baluncore]

Surface velocities are important with bearings. For that reason some scale is needed.
1. What is OD of red shaft?
2. What is ID of air flow tube?
3. What is differential air pressure?
4. What is side forces on red shaft?
5. What is kW rating of motor?
6. Why does the red shaft need to contact the air in the tube?
Could it not just pass without contact through a stationary tube that is firmly attached to the air tube.
7. Is it the shaft of a butterfly valve or choke?
8. Why is there oscillatory motion?
9. Is there any axial movement of the red shaft?
Air pressure will blow lubrication out of a bushing, or moisture and dirt into a ball race. You need to use two or four lip seals on the red shaft. If you use ball bearings you should specify rubber lip seal bearings suffix -RR or -2R.

 

[Mormont]

Hello Baluncore,

Thanks for taking time on this, below are some details;
1. OD of red shaft is around 8mm
2. ID of air flow tube around 20mm
3. Need to design a low pressure loss valve, I suspect the loss will be around 5 - 10 cmH2O
4. Side forces on red shaft would be the force due to air pressure which is about 100 cmH2O
5. Rating of motor is about 10W
6 & 7. Yet to decide between butterfly or globe valve
8. Oscillatory motion for required application (max. 90deg at 30 Hz)
9. Nope
Can I use ball bearings with a cover, as in link below;
https://www.vonarx.nl/userdata/artikelen/von-arx-fr200-ball-bearing-cover-side_248_1_G.jpg

Hope this helps for you to help me!

 

[Baluncore]

Quick comments;

Avoid rolling bearings for reciprocating movement as lubrication is poor. They will bruise or brinell due to short reciprocating motion. https://en.wikipedia.org/wiki/Brinelling

Bushes may do better in your application as they have a large surface area. Look at carburettor butterfly valves for guidance.

You have pressure not depression so you may need a lip seal at the inner end.

Alignment of close fitting bushes is critical so do not clamp them between separate parts. Press the bushes into a fixed housing then ream them in alignment to make a valve body. Where clamped bearings are used they must be self aligning bearings such as spherical roller bearings.

If the shaft is highly polished you might use Teflon bushes rather than brass. But every polymer absorbs water so clearances might then be a problem. Temperature also effects polymer clearance dimensions more than brass.

Note that an elliptical butterfly will seal a round tube, but it will also have a hard stop and so need a spring loaded friction coupling to prevent damage at the angle where it seals.

 

[Mormont]

Quick comments;

Avoid rolling bearings for reciprocating movement as lubrication is poor. They will bruise or brinell due to short reciprocating motion. https://en.wikipedia.org/wiki/Brinelling

Bushes may do better in your application as they have a large surface area. Look at carburettor butterfly valves for guidance.

You have pressure not depression so you may need a lip seal at the inner end.

Alignment of close fitting bushes is critical so do not clamp them between separate parts. Press the bushes into a fixed housing then ream them in alignment to make a valve body. Where clamped bearings are used they must be self aligning bearings such as spherical roller bearings.

If the shaft is highly polished you might use Teflon bushes rather than brass. But every polymer absorbs water so clearances might then be a problem. Temperature also effects polymer clearance dimensions more than brass.

Note that an elliptical butterfly will seal a round tube, but it will also have a hard stop and so need a spring loaded friction coupling to prevent damage at the angle where it seals.

Hello Baluncore,

Can bushing be a safe option if my speed has gone down to 700 RPM from 1500?

Thanks!

 

[Baluncore]

8. Oscillatory motion for required application (max. 90deg at 30 Hz)

Can bushing be a safe option if my speed has gone down to 700 RPM from 1500?

Without some idea of the application it is difficult to know what would be acceptable.
Is it undergoing full rotations or is it oscillating through a maximum of 90°?
Is it only air that is being controlled, why are you controlling it?
Any contaminants or condensation in the air or environment will raise issues.

Can you apply the higher pressure air from upstream to the outside of the bushes, to balance the pressure and so reduce flow through the bushes, flow that would otherwise blow dust in and lubricant out?

Are you sure you can operate the globe or ball valve at that speed? There will be transmission line effects in the plenum, both up and down stream of the switching valve. Where fast pneumatic control is required it is normal practice to use dry compressed air with a solenoid valve.
Again, without some idea of the application and environment it is hard to say what is acceptable.