Best Fix For A Worn Axle?

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Lood

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I recently replaced a 70kw electrical motor with a new one, due to the worn out axle of the old one. The old motors' fan broke, for some unknown reason and it overheated substantially. I replaced the fan and bearings, but ever since, it had a slight vibration, which got worse over time, to such an extent that the motor would draw too much current and trip the O/L.
The problem is that the rear bearings' inner does not fit tight enough over the motors' axle after the over heating event and that's where the vibration takes place. The bearing can actually be moved slightly by hand! I fitted a new bearing and I tried to fix the problem by adding shim stock, but it only worked for a day or so.

Seeing that the motor doesn't really have anything else wrong with it, I'd like to fix it. A friend of mine suggested that I turn the axle down on my lathe and then turn a sleeve, which is pressed onto the axle and then machined down to the exact required diameter to ensure the proper fitment of the bearing.

My questions are:
1. Will my Graziano SAG180 be able to handle the load? I haven't weighed the armature, but it is extremely heavy, with a 70mm axle.
2. If the lathe can handle the load, should I opt to use my 300mm 4 jaw chuck, instead of the smaller 3 jaw?
3. What size live center should I use at the tail end?
4. What should the recommended wall thickness of the sleeve be and what material should I use for the sleeve?
5. What is the recommended tolerance between the bearing inner and the outer diameter of the sleeve, to ensure the proper fit of the bearing?
6. What is the recommended tolerance between the sleeve and the axle, to ensure that the sleeve stays put and not start moving around the axle? Should I opt to add some sort of grub screw to prevent any movement of the sleeve?
7. Should I rather have a qualified shop do this job, because I can just imagine the chaos that would erupt if this job is not completed extremely precisely?

Thanks in advance for your feedback.
 
The axle on a powerrake had this problem [pulley had spun on the shaft a bunch, wore it down], and my dad fixed it by brazing the worn part, then using a lathe to get it back to the original size.

I have no experience with doing this myself, so I can't answer your questions.

Hm, maybe when I get a mill, I might put a slot for a key in that axle...I'll have to research if I should do it though, as it's driven by a 7HP gas engine and the axle has a pretty good rotating mass.
 
1. Will my Graziano SAG180 be able to handle the load? I haven't weighed the armature, but it is extremely heavy, with a 70mm axle.

While not familiar with that particular machine, I can't say for sure, but generally if a machine will swing the diameter you ask, then the weight isn't a major factor, particularly if it is balanced fairly well.

2. If the lathe can handle the load, should I opt to use my 300mm 4 jaw chuck, instead of the smaller 3 jaw?

Yes

3. What size live center should I use at the tail end?

As large as possible, as this will likely be the real limiting factor in handling the weight of the part. If you have access to the specifications on your center, check it for weight capacity.

4. What should the recommended wall thickness of the sleeve be and what material should I use for the sleeve?

Generally, any mild steel will work for your application. Nothing special about it. It's common to use Stressproof for shafting itself, but for a repair sleeve, nearly anything will work.

5. What is the recommended tolerance between the bearing inner and the outer diameter of the sleeve, to ensure the proper fit of the bearing?

See technical data at the bearing manufacturer's website. Or most any reference chart regarding bearing fits.

6. What is the recommended tolerance between the sleeve and the axle, to ensure that the sleeve stays put and not start moving around the axle? Should I opt to add some sort of grub screw to prevent any movement of the sleeve?

Press fit 0.001-0.003 under prepared (turned) bearing area. Use a generous radius in the corner where the newly prepared surface meets the existing face/shoulder. DO NOT move the shoulder back. On the ID of the repair sleeve, chamfer the ID to clear the corner radius on the shaft. No screws needed if the press fit is correct. Finish machine to spec after pressing the sleeve on. If a press is not available, reduce the interference and use heat to expand the sleeve and wasting no time, slip it over the shaft.

7. Should I rather have a qualified shop do this job, because I can just imagine the chaos that would erupt if this job is not completed extremely precisely?

This all depends on how confident you are in your skills. Stateside, a process known as metalizing is commonly used for shaft repair as you seem to need. I have done many back in the 70's. Steel, chrome, aluminum and many other metals are sprayed onto other surfaces by the process and it can be used to build up shafting and hydraulic rams, etc.. You will need to be able to control your runout and size very closely to come out with a safe, reliable repair. This is where your own judgement will be your guide. If spray buildup is not an option, then a sleeve is perfectly acceptable.
 
Sleeve or weld shaft, time consuming and expensive. Nowadays, metal spraying is the choice of most motor rewind/repair shops. I've had them save several old large (10HP & greater) hard to find motors with metal spray and OD grind to fit and rewind/redipp windings.
 
jb weld and turn it back to proper size, a cheap fix and it will work
 
Given the questions you're asking I suspect this is a job you may want to have done by a shop that specializes in such work. I have sleeved large (not THAT large) motors before and it needs to be turned between centers. So you would need a large enough lathe dog for the motor shaft, face plate, spindle center, etc.

I've also found that loose bearings can work harden their shafts considerably making turning difficult. Tolerances can be determined from Machinery's Handbook but those are no substitute for field knowledge of the application. That's a LOT of motor to experiment with.

As Fabrikator mentioned "spray welding" has been used to build up shafting for small increases in OD with good success. It certainly involves a lot less machine and press work.
 
This might be a long shot but it worked for me. I have a sleeve bearing motor that had play in the output bearing. I removed
the sleeve bearing and pinched it down a few thousandths in my 4 jaw chuck, reducing the clearance. It works fine
now. It's an old brushed motor with sleeves, a dipping ring and a reservoir. I hope this information is useful to you...
 
I agree with @Tony Wells and @mws. Metal spraying or sleaving is an acceptable method of repairing the axle. But..... that is only half of the job. Given that you are working with a 70kW (~100 HP) motor and it overheated, the windings need to be checked out also. Unless you have the proper equipment and the knowledge to do this, I would be inclined to take to motor to a qualified shop to have the work done.
 
Jim, you bring up a valid point. Original problem seemed to occur when the fan broke. As you know, many of them are simply cast aluminum or zinc alloys. Fractures are not uncommon. If the overheating was only because of the lack of a fan, the damage isn't likely to be too severe, but certainly bears checking out.

The way I was taught, I used a threading tool or similar, to "rough up" the surface to be built up. Turning a little undersize prior to this is necessary because most of the spray alloys in use for this application come out more or less like a powdered metal product, or sintered, and are not that "tough" and shouldn't be less than around 0.030-0.040 thick at finish. After scarifying the surface, I used a very thin pure nickel coat for proper bonding. Then the top coat is applied and obviously generously enough to be machined back to finish without voids and flaws. On shafts I have had to repair the shoulder/face on, it's important to undercut the shoulder a bit to make sure the sprayed metal has a hook into the parent material. This all can get a little tricky if you are on the clock doing it. It gets a bit warm, as you might imagine, and hitting the cool size on a hot shaft is not easy.

Metalizing is an interesting process, and I have been surprised at how well some of it turned out. I couldn't tell you how many pounds of wire I have sprayed, but many. I don't have the metalizing rig in my shop (too dirty anyway), but I do have a powdered metal oxy/fuel buildup torch. It works for applying hard facing to relatively light pieces. It is not intended for big shaft work like the OP has. The shaft would need to be near welding heat through, and puddle on the surface. Not practical.

For fun we used to spray the metal across the shop at someone. It's not unlike what comes off a cutting torch.:)
 
Turn damaged shaft od to known size to give a reasonable sleeve thickness. . Make a shrink fit sleeve and fit. Then Turn sleeve od to suit bearing.
 
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