M936A1 rear deck winch failure

[SLOrazorsedge]

During a winch pull using my M936A1 heavy duty rear deck winch attempting to pull a bay laurel stump, the winch stopped under load and then released the drum into a partial bird nest. We were able to unwind the bird nest and rewind the 100 feet of cable back in as long as there was minimal load on it. Anything more than a man pulling up slack the winch would stop rewinding. There was no loud bang gear crash or noise other than the sensation of a clutch releasing/failing. If a shear pin let go, I would think I wouldn't have been able to recover my cable. Any similar experiences or ideas on where to start? The truck was a 2010 rebuild out of Barstow and everything looks new on the truck.

 

[212sparky]

Check the hydraulic motor. You may have head the veins in the motor fragment causing the motor to limp. It would idle but once you put a load on it stalls. Had one do this on our line truck, on the auger motor. Good luck.

 

[SLOrazorsedge]

Ouch, I hope not. Does the hydraulic motor pump to the crane and front winch too? The only noise seemed to be the winch releasing into a backlash.

 

[m16ty]

Check the shear pin first. It could be broke but there could still be enough resistance in the coupling to spool in the line without load. The motor on these things are massive so I'd say it's unlikely it broke.

Hyd motors don't pump, pumps do that. If you screw up your winch motor the crane and front winch will still work (the front winch is a whole different system). If something breaks inside the motor it could screw up your whole wrecker hyd system if it sends broken pieces down the line.

You don't seem very familiar on how the whole M936 system works. I suggest you read up on the TM and understand how everything works before you try and repair it.

 

[SLOrazorsedge]

Thanks for your reply. We've been all over the TM's, I printed the ops TM and we had it out during operation. This was the first run on the deck winch. We've run the front winch and crane a number of times w/o issue since we bought the truck last June. We had the truck tethered to an oak tree on a straight pull. One stump pulled beautifully. The second stump 16" in soaked ground budged only. I never dreamed it could exceed 45,000#'s. If it is the shear pin, is that a major debacle to R&R? I'm going to drain the 3qts of oil out of the winch tomorrow and check for broken parts. Got my head buried in more TM's all night.

 

[zebedee]

Your symptoms sound very deja vu ... Had the same thing happen last summer (see thread "what have you done WITH your wrecker") but on my M816 which is all mechanical - no hydraulics (on the winches).
Think you are safe to say it's shear pin - however I'll not be fixing mine 'till it gets back above freezing!

 

[rangereter]

Before you drain your oil, you should get under the winch, have a helper operate the wind-in control (with your cable attached to an anchor), look up at the winch drive coupling (big double bike chain looking thing about 3" in diameter) and see if both input and output spin together to determine if you broke the shear pin. I think the shear pin is part of the coupling. Regards, Bob

 

[73m819]

shear pin

 

[SLOrazorsedge]

Thanks guys for the replies. I'm starting to breathe a little easier that it might be the shear pin. I'll be climbing under the beast this morning for more diagnosis. Reviewing the TM's last night I could not find where the shear pin was located, either internal or external. Rangereter's suggestion makes sense and will be first plan of attack. Still kicking myself for "powering thru" when the winch slipped on the first pull....

 

[73m819]

The shear pin is right before the chain coulping on the winch side

 

[JH1]

All hydraulic systems have some sort of relief valve somewhere. If the spring broke inside the relief valve, it would not let much pressure build before releasing it away. However, if other hydraulic systems work properly, then you have all the correct high pressure, and the relief valve wouldn't be the problem. Sometimes, the relief valve is located inside the hydraulic control. The relief valve should protect the winch, assuming it is the only hydraulic motor in the sytem, but if you have mulitiple hydraulic motors that create different torques, then each motor output would have it's own shear pin, sized appropriately for that particular output duty.

 

[crasheej]

Were you useing one line to pull stumps? If so it is no wonder you sheared the pin, get snach block . That wiil help to lesten the load on the wench, make it safer for the operater.

 

[SLOrazorsedge]

Update: My 31YO son and I spent the afternoon on our backs under the wrecker. Our hearts sank when the shear pin appeared intact. Closer exam noticed the round head of the pin was backed out about 1/4". My kid put some pry pressure on the head which popped right out leaving the long shank of the pin with cotter pin remaining in the collar. The remaining portion of the pin must have swelled at the break point as the input and output shafts will still turn together. The downside is the collar turned on the shaft out of sequence from the pin hole so we can't drive the pin out. No matter how many times we turned the shaft chain drive by firing up the truck and bumping the winch drive or alternatively by hand we could not line up the pin hole with the collar. We pried, twisted and tapped the pin...finally we drilled the center of the pin out approx 3/4 inch and threaded a bolt into the opening. We then welded the bolt to a small slide hammer shaft, threaded the tool into the shear pin and managed to extract about 3/8" before the weld broke. We repeated the bolt weld and again slide hammered the pin about another 3/8" until it seems stuck and broke our weld again. We'll try again next weekend weather permitting. Thank you again for all your suggestions, m16ty, ranereter and 73m819 you guys were spot on! Anyone know where I can source shear pins for the deck winch? It appears to be unique having a steel round head with stepped shoulder on the piece we did extract.

 

[wreckerman893]

Were you useing one line to pull stumps? If so it is no wonder you sheared the pin, get snach block . That wiil help to lesten the load on the wench, make it safer for the operater.

Mechanical advantage is your best friend for using a winch. There is a TM concerning vehicle recovery that addresses how to rig for pulling. Maybe someone knows which one it is and can post a link to it. Bear in mind that with a straight line pull if something gives and that cable comes loose it will cut a man in half if it hits him right. Be very careful during winching. We were required to always wear our Kevlars when we did it.

 

[SLOrazorsedge]

Thanks wreckerman for the tip. We did blanket the line with a 100# chain midway and I was down below the wrecker deck winch controls, my kid was laterally 50 yards away behind an oak tree. I would appreciate directions to the vehicle recovery TM. My truck came loaded with a complete BII, inventory check list , 6 snatch blocks starting at 10 ton and the whiffle tree. The tool boxes were all sealed with cable locks thus no inspection pilfering. What's interesting about the truck is it was inspected and sealed in Mississippi in 2010 and shipped to Barstow. No unit markings on the truck appears it was not issued at least since rebuild in 2010. I watch for and appreciate your threads for learning.

 

[zebedee]

Go to "Technical Manuals" under the Steel soldiers banner,
Scroll down to "Misc TMs"
Scroll down to "TMs for Towing"
Got to FM 20-22.

NB. This is a generic manual but should cover all the situations for your vehicle when read with your own vehicles -10 TM.

Please read this thoroughly, then come back to the forum if you don't understand any part of it.
There are plenty of experienced people here who can give you reasoning and examples of why these instructions need to be followed for everyones safety - let alone prevention of damage to equipment.

Be safe... : )