cantilever shaft reinforcement

[safecracker]

I am looking for comments or experiences regarding an idea I have to reinforce an existing drive.
The drive consists of a Vbelt pulley mounted on a shaft supported on each side by selfaligning bearings, one a ball bearing and the other a double row roller bearing. Outside of the double row roller bearing is a sprocket that transmits power to the rest of the system. The problem is that we have been having failures (fractures) of the shaft at the roller bearing on the pulley side. I am sure it is because of the bending stress on the shaft. The design of the machine does not permit a major redesign so I am attempting to reinforce the existing design.
My idea is to drill a hole down the center of the shaft from the sprocket end into the middle of the pulley with the end of the hole tapped with a fine thread. Then I plan to install a high strength bolt in the hole and torque it up, the idea being that it will put compressive forces in the shaft and help to relieve or diminish the tensile forces on the surface of the shaft, thereby improving the fatigue resistance of the shaft.
Any thoughts/comments will be appreciated.

 

[MASSEY]

Check the thread called:

"fatigue failure of a low stressed shaft"

It is in the "Metal and Metallurgy engineering" forum.

 

[macmil]

What is the shaft material ? Go to a higher specification material with better properties especially tensile, toughness and fatigue resistance. If you need more info feel free to contact me at 403 278 7604
mac

 

[safecracker]

The shaft is made of heat treated 9310, a nickel chrome moly alloy. It should have good fatigue resistance, similar to 4340

 

[MadMango]

Can you weld or mechanically attach a larger OD sleeve to your shaft? Sounds like you are getting stresses related to torgue. Increasing the shaft diameter should yeild better torque resistance. "The attempt and not the deed confounds us."

 

[bhwtam]

Maybe I am talking stupid, but is it possible for you to change a higher load rating bearing instead?

 

[safecracker]

The problem is not with the bearing. However, Madmango is alluding to another thought that I have for reinforcing the shaft. The bearing is a press fit onto the shaft and that is all that is holding the shaft in position axially. The shaft has a larger diameter center section where the pulley is fitted. My other thought was to fit a close tolerance sleeve around the shaft between the raised pulley hub and the inner race of the bearing, then fitting another sleeve over the shaft that butts up against the other side of the inner race and extends just past the end of the shaft. The end of the shaft would be drilled and threaded for a bolt. A thick washer would be fitted around the bolt and would bear up against the end of the sleeve and would be torqued up to put the shaft in tension and the sleeves in compression. During operation, the compressive loads generated by the bending force would be applied to the sleeves and the tensile loads generated by the bending force would be taken up by the shaft. The idea as such would see added tension on the shaft but without the stress reversals every 180 degrees of rotation as with the present setup. However, I have been told that it is not good to run a shaft in tension. Your thoughts?

 

[christoph]

FYI. Thread referenced by MASSEY, above, regarding "fatique of shaft" is thread330-21601 (long). I didn't check, myself, to see if the issues therein are related to your specific problem.

 

[stressriser]

My first thought, since you asked, is that you suspect "Bending Stress" as the problem. You then state that you plan on fixing it by adding "Tensile" strength to the shaft.

These are two different "Stresses" and you are not going to fix bending stress by adding tensile strength. The shaft only has tensile bendin one one side of the neutral axis. Further thought on this tells me that you may make the problem worse.

If you suspect the Bending stress is the culprit, you verify this by calculating the bending moment. The deflection of the shaft from the pulley and the sprocket are calculated. The angle of twist is calculated. The torsional twist is calculated.

Once you do all this, you need to determine the maximum stress, which is the principal stress, there is no axial (longitudinal) stress so you just use the bending stress and the maximum torsional shear stress in the calculation.

Then you use the Distortion Energy Theory to find the von Mises stress.

Then find the ultimate strength of the material, see if you even have a safety factor.

You will not ever be able to fix it properly unless you run the numbers first. It sounds like you may be fixing the problem without even knowing what it is. Usually, it is torsional stress that breaks shafts, but since you have BOTH torsional and bending stresses, you can't fix it by adding tensile strength, since there is not even any axial stress.

 

[MASSEY]

I notice you said the inner pulley is on a larger diameter than where the bearings are mounted - I assume you have considered the stress concentrations where the diameters change, and design of these transitions is critical.

Have you considered mounting a friction clutch where power is transmitted to the system? This would apply your torque gradually, reducing both stresses.

 

[vonlueke]

Adding a sleeve won't help if most of the stress causing the failure is torsional shear, because the sleeve ends won't be connected where the shaft is breaking. And welding these ends might degrade the shaft material (?).

Bending and tensile stress are additive. You are correct; precompressing the outside fibers of a rod reduces the total tensile stress, whether generated by tension or bending. (Compressing a rod, however, can amplify bending moment from transverse loads, unless transverse deflection of shaft is virtually zero.) Run the analysis per stressriser, including stress concentration factors at transitions, to investigate your ideas, and to see if bending truly dominates.

Even though you entitled this thread "cantilever shaft," it sounds like you instead have a beam simply supported on one end and fixed on the other (at first row of roller bearings). Correct? Consider using a larger-diameter shaft without transitions, sized appropriately based on the analysis. Or if bending truly dominates, perhaps consider adding a double row of roller bearings where the ball bearings currently exist; and of course shorten the shaft as much as possible. Furthermore, use extremely generous fillets at transitions; and eliminate all sharp reentrant (interior) corners in the shaft. It's possible you may find most of these measures fairly ineffective compared to increasing the shaft OD.

 

[safecracker]

Thanx for your replies. The original shaft was 30 MM diameter and ran a triple row # 50 chain on a 19 tooth sprocket that is just outside the selfaligning double row roller bearing. This sprocket has a pitch diameter of 3.719". The chain drive was replaced with a Gates Polychain 8MM belt drive running a 62MM wide belt on a 34 tooth sprocket which has a pitch diameter of 3.4". Because the belt drive sprocket is smaller than the chain drive, the tension in the belt is higher than the tension in the chain was for a given torque. There has been an increase in the number of failures of this shaft with the belt drive as compared to the chain drive. This leads me to the conclusion that it is bending stress that is breaking the shaft and not torque as the torque has not changed.
The shaft was increased to 35MM diameter which is about all that will fit in the application. So far there has been one failure with the belt drive on a 35MM shaft.
I have designed a larger belt drive that has a drive pulley of 4.3" pitch diameter that should reduce the tension in the belt below that of the 34 tooth drive as well as the chain drive. I think it should help.
The reason I posted the original question was that I was thinking of a way to reinforce the shaft as it would be cheaper than fabricating a whole new belt drive and wondered if anyone had had any experience with the idea.

 

[MASSEY]

safecracker,

Have you considered installing a plastic "doughnut" between the drive pulley and bearings.

I forget the correct name for these plastic couplers - we used them in a cement plant on drag-line chain driven shafts. They are designed to flex and slightly twist so all the stresses from torque and bending are not transmitted along the shaft. They are "absorbed" at the coupler.

http://www.naismith.com.au/index.html

pictures some different styles. Good Luck.

 

[ttdmt]

Hi safecracker,

I read your thread about chain to belt drive conversion. I am designing a fan drive with Gates Polychain. We are an OEM supplier to the frieght locomotive business and have never used this technology before. We are not sure how long they will last in our invironment, how many hours do you get on your drives? We also have installation/maintenance questions. How hard are these drives to install, align and tension properly? Do you go back and check tension? If so how often? What tools to you recommend for accurate alignment and tensioning?

Do you know of any independent experts in belt drive technology?

 

[safecracker]

hello ttdmt
Have you got a copy of Getes' Polychain Drive design manual? If not, you can get one from your local Gates dealer or search the net for Gates. They have a drive design program that you can download.
As far as life expectancy is concerned, I have not been able to find any information other than "indefinite if installed according to thier recommendations". Apparently they do not like to be exposed to sunlight or ultraviolet light as that will deteriorate the belt as will certain chemicals. As far as installation is concerned, the shafts should be within 1/4 degree of being parallel in both planes. I am using a digital electronic level but you can use standard shaft alignment procedures. The shaft alignment is quite critical as misalignment will cause uneven stress distribution in the belt and may lead to tearing in the back of the belt. Also, I discovered that there is a "right hand rule" for these drives which states that if you lay the fingers of your right hand on the belt with your fingers pointing in the direction of travel, the belt will normally tend to run on the side of the pulleys that your thumb is pointing to, even up against the antiexcursion ring. The belt will generally require retensioning once after installation and will thereafter be essentially maintenance free. The proper tensioning of the belt is critical to proper operation and Gates' drive calculator program will give you the specs for tensioning the belt or consult with your Gates dealer as they can get the information for you. In my application, I am having the sprockets made up as off the shelf components are not available. They are being made with removable antiexcursion flanges to make belt installation and replacement easier. My application does not have very much adjustment for belt tensioning, ie, the range of adjustment is such that the belt cannot be installed over the antiexcursion flanges when installed. The Gates polychain drive is excellent and I am sure you will be pleased with the results.

 

[sprintcar]

Be careful with the Polychain designs. They're ok if you're replacing metal chain, but if you use them instead of V-belts you have a severe increase in noise to consider.

I worked on the Harley beltdrive for the '85 models and alignment was the most critical issue - it has to be EXACT in all directions. Drive guards can become noise amplifiers so take that design into consideration as well. Keep the wheels on the ground
Bob
showshine@aol.com