Bobcat 731 drive motor identification??

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campbell45

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Sep 16, 2023
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32
Hi guys, hope someone can help me.

I recently bought a used bobcat 731 for pretty cheap. Runs and drives but soon after running the right side would not want to move. I assumed it was the hydrostatic drive pump instead of the motors.

I have sent the hydrostatic drive pump to be rebuilt and decided to pull both drive motors to put a new seal kit in and inspect them for wear.

The left side (which let the machine move easily) looked original and so I tore that down first.

Afterwards, I tore the right side down (which was the side that did not want to move). I discovered a few differences in this pump compared to the left side. See the attached picture.

As you can see the right side has more of an oval shape at the bottom where the o-ring sits compared to the left which is a complete circle.

I'm now worried someone replaced my right side with an incorrect drive motor. Or possibly this is a revised version of the original drive motor? Can anyone help me identify this? Both motors are missing the riveted ID tag.

drive motors.jpg
 

Tazza

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Dec 7, 2004
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16,844
Talk to loader parts source, it's an eaton drive motor, they will have a seal kit for it.
 
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campbell45

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Sep 16, 2023
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Thanks Tazza. I already have the seal kits for it. I'm wanting to understand the difference between the two motors. Has anyone seen a motor like this?
 

Tazza

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That is weird, i don't know why it does that to you....

As for the other motor, that is a weird one, i have never seen one of them that is not just a round cut.
 
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campbell45

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Sep 16, 2023
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32
@Tazza How do I go about fixing this so that the rest of the forum can see my posts? Is this something you can fix being a staff member? Thanks for the help if so
 

mikelv

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Sep 9, 2023
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58
must be ok now. I can see the post. sorry I dont have any answers for you about your pump thought.
 

River Fluid Power

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Aug 18, 2022
Messages
112
Hi guys, hope someone can help me.

I recently bought a used bobcat 731 for pretty cheap. Runs and drives but soon after running the right side would not want to move. I assumed it was the hydrostatic drive pump instead of the motors.

I have sent the hydrostatic drive pump to be rebuilt and decided to pull both drive motors to put a new seal kit in and inspect them for wear.

The left side (which let the machine move easily) looked original and so I tore that down first.

Afterwards, I tore the right side down (which was the side that did not want to move). I discovered a few differences in this pump compared to the left side. See the attached picture.

As you can see the right side has more of an oval shape at the bottom where the o-ring sits compared to the left which is a complete circle.

I'm now worried someone replaced my right side with an incorrect drive motor. Or possibly this is a revised version of the original drive motor? Can anyone help me identify this? Both motors are missing the riveted ID tag.

View attachment 4699
These are Char-Lynn motors. Either 4000 or 6000 series. I would need widths on these valve plates, or photos of the end caps to confirm which.
 
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campbell45

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Sep 16, 2023
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@River Fluid Power is this what your looking for? I can take more if needed. These pictures are of the one I question is not original. Thank you for the help!
 

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campbell45

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Thanks @River Fluid Power! The wear is not terrible for the motor on the right. The slight grooves seen on the mounting flange are very minimal - enough that I feel comfortable cleaning it up with some glass and sand paper. Unless you're seeing something considerable I'm not - or advise against that??

However, the motor on the left (pictured earlier and the one I believe is original) definitely has enough wear that I think oil pressure could be lost.
But I did not notice any drive issues with this side - I'm sure theres got to be some loss of power due to the wear though.
Are you able to get individual parts like the geroler and valve plates?
Also have you seen any 4000 series valve plates with that kind of oblong circle thats pictured on the right?
All the 4000 series I've seen are a perfect circle for the o-ring.

My local hydraulic shop had the ability to get parts and rebuild my vickers TA1919 but said they arent able to get parts anymore for these drive motors... and in theyre words, if they could, it wouldnt be economical to replace parts vs buying a whole new drive motor.
They also mentioned bobcat usually specified custom displacements.
For example, the original drive motors may have been spec'd at 230 cm3/r for these bobcats.
But char-lynn only shows either 205 or 245 cm3/r displacements.

Your thoughts? and again, thank you!
 
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campbell45

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Sep 16, 2023
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32
Thanks @River Fluid Power ! The wear is not terrible for the motor on the right. The slight grooves seen on the mounting flange are very minimal - enough that I feel comfortable cleaning it up with some glass and sand paper. Unless you're seeing something considerable I'm not - or advise against that??

However, the motor on the left (pictured earlier and the one I believe is original) definitely has enough wear that I think oil pressure could be lost.
But I did not notice any drive issues with this side - I'm sure theres got to be some loss of power due to the wear though.
Are you able to get individual parts like the geroler and valve plates?
Also have you seen any 4000 series valve plates with that kind of oblong circle thats pictured on the right?
All the 4000 series I've seen are a perfect circle for the o-ring.

My local hydraulic shop had the ability to get parts and rebuild my vickers TA1919 but said they arent able to get parts anymore for these drive motors... and in theyre words, if they could, it wouldnt be economical to replace parts vs buying a whole new drive motor.
They also mentioned bobcat usually specified custom displacements.
For example, the original drive motors may have been spec'd at 230 cm3/r for these bobcats.
But char-lynn only shows either 205 or 245 cm3/r displacements.
https://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1333.pdf

Your thoughts? and again, thank you!
 

River Fluid Power

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Aug 18, 2022
Messages
112
You're welcome @campbell45 . You definitely want to clean up as much of the scoring that you can. I would be concerned about the smearing I see on the left plate. The oblong o-ring groove is how the older plates were made, it is interchangeable with the plate with the round groove. Individual parts are available, but cost and lead time is usually a factor on Char-Lynn parts. It is sometimes cheaper to buy a complete motor. The trick is finding the exact motor you need. I would say the displacement of your motor is 9.9cid, or 160 cc's. I can verify that with a photo of the geroller assembled.

1695902009403.png
 
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campbell45

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Sep 16, 2023
Messages
32
Thanks River. This help and info was extremely valuable to me - especially the oblong circle being an earlier design. I took a look at your shop. Any upcoming rebuilds, I will reach out to you for parts/service. Again, thank you!

Do you happen to know how much material I can safety remove from the plate surfaces until binding occurs in the valve drive and drive "axles"? It seems any gap tolerance would be taken up by the balance ring and springs once mounted. I guess I can start measuring to know how much I can take off before things start to bind...If I had to guess I'd need mill 1/64" off (as well as the same distance in the oring channels)
 

River Fluid Power

Well-known member
Joined
Aug 18, 2022
Messages
112
Thanks River. This help and info was extremely valuable to me - especially the oblong circle being an earlier design. I took a look at your shop. Any upcoming rebuilds, I will reach out to you for parts/service. Again, thank you!

Do you happen to know how much material I can safety remove from the plate surfaces until binding occurs in the valve drive and drive "axles"? It seems any gap tolerance would be taken up by the balance ring and springs once mounted. I guess I can start measuring to know how much I can take off before things start to bind...If I had to guess I'd need mill 1/64" off (as well as the same distance in the oring channels)
You're welcome. Of course I would be happy to help you with parts in the future. .015" doesn't seem like a lot of material to take off, but typically the rule of thumb for reworking flat parts in Avila piston pumps and motors is .005" - .015" overall. In the case of these disc valve motors it will probably be ok. I don't think it will bind, however. I think if you took too much material off the valve would be pushed off of the valve plate, thus losing the seal between the two pieces. That means you won't be going anywhere. Keep in mind that these pieces hard most likely case hardened, so removing too much material could lead to the metal being too soft. That leads to smearing and failure.
 
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