Winch Mount Bolt Strength

[Retired Tanker]

Has anyone ever sheared a winch mounting bolt while winching?

I'm asking because I'm fabricating a hidden winch plate and need to make sure I've got enough strength in my attachment points. After thinking it through, it seems that the weakest point in the whole equation is where the winch is mounted to the plate. My 12000 lb. winch has Metric M10x1 bolts with 8.8 on the heads. But I suck at converting MegaPascals to something useful, like can I pull myself out of a hole...

But, if these bolts are strong enough to hold the winch to the plate, then there's really no reason to over engineer the bolts that hold the plate to the vehicle...

Am I missing anything here?

 

[Hillbilly Heaven]

According to this chart to get to an equivalent Grade 8 imperial strength you would need 10.9 bolt. Not a good image but usable.

 

[DaveInDenver]

Seems you need to tell us how you're loading your bolts. Most winches are mounted feet forward so the bolts aren't under much strain, particularly in a straight pull. If you've mounted feet down the bolts are in shear (e.g. the plate wants to cut them off). Feet forward puts the mount into compression, which isn't straining the bolts beyond the off axis load.

Generally pascal is a measure of pressure, which is force per area. One pascal is by definition one newton per square meter, e.g. 1 Pa = 1 N / m^2. The imperial unit that is similar is lbs per square inch, e.g. psi.

What this means to a fastener is you find its area and multiply that by its psi or pascal rating. Metric bolts give you their ratings directly, 8.8 meaning tensile strength<dot>yield strength. IOW, an 8.8 means 800 MPa and 80% of tensile, e.g. 640 MPa is it's rating. A 10.9 is 1000 MPa and 90%, e.g. 900 MPa.

If you convert 640MPa to psi you'll find that to be 92,824 psi, which is as noted basically the same as an SAE grade 5.

Next you find bolt area, which is just geometry. A 10mm bolt has radius of 5mm or 0.005 meters and area = pi * r^2, e.g. it's 7.85 x 10-5 m^2.

So a 10mm 8.8 bolt will handle 7.85x10^-5 m^2 * 640 x 10^6 N / m^2 = 50,240 N or 50.240 kN.

Newtons are measure of force, which can be converted to pounds-force by dividing by 4.448, e.g. that bolt can handle 11,294 lbf. Bolts are rated in tension, so if you're using them in shear you have derate them to 60% usually. That means in shear a 10mm 8.8 will take ~6,777 lbf working.

To know exactly how strong the mount is you need to figure the load on the bolt pattern and how they are loaded. It's not necessarily four times each individual bolt, though it will be higher than each individually. You can assume Warn (or whomever) has done the maths and knows 4 x 10mm is strong enough but you'd have to know what grade they assumed.

If you use 4 x 10mm grade 12.9 then you'll be sure it meets or exceeds the winch bolts themselves, though it's not really that simple. The area of the clamped interface factors in, IOW when you bolt a connection the bolt itself is providing a clamping force and the friction of the two surfaces is what is resisting movement. So the bolt doesn't necessarily have to resist the full force.

 

[Retired Tanker]

Thanks for the great research and calculations, Hillbilly and Dave. I tried to work through them, but it was making my head hurt...

The winch is definitely a "feet down" design, with nut inserts for the interior threads rather than threaded into the mounts themselves. I think that going up a grade or two on the bolts will help with shear forces. Not sure if I can find inserts that match the grade, but I don't think it will matter that much. There is minimal leverage angle impacting the shear on the threads.

I hope the manufacturer (It's a Havoc winch that I think is manufactured by Engo) based their calculations and engineering on the materials that they supplied!!

 

[1stDeuce]

As a mechanical engineer, I recommend against using 12.9 bolts, pretty much ever... They can suffer from hydrogen embrittlement over time due to exposure to the environment, and can then fail basicall just sitting there. They are used in some instances, but usually only in protected environments, and their application is getting more and more rare... 10.9 is the highest grade that should be considered for general bolt needs. It is a bit higher than a Grade 8 standard fastener.

As for your bolt questions, the M10 8.8 bolts that came with your winch are fine for holding it. Understand that bolts are almost NEVER used in shear, even when the load appears to be in a direction that would cause shear. Shear would only be the case if the bolts were left loose. The whole point of torquing bolts to a specification is to obtain a desired clamp load at the joint. It is the clamp force that holds any load that the joint sees. If the joint slips, and causes the bolt to see shear stress, then the torque was too low, or the bolt was undersized for providing the amount of clamp needed at proper torque.

In the case of your M10 8.8 bolts, you'll get almost 6000 lbs of clamp load at 31 ftlbs dry (zinc plated). There are four, so you're looking at somewhere around 24,000lbs to move the winch. Even if you assume the cable is all the way at one side, you'd still have to get to 12,000lbs on that side to slip the feet. (Assuming a static friction of ~1, which is probably about right for a bolted connection.

You could use a higher strength bolt, but since you have to assume that the nuts are also grade 8.8, you still can't install them to a higher torque, since you'll destroy the threads in the nut.

Just put it together with what they gave you, and if you do one thing to ensure a proper mount, use a torque wrench and NO lube on the threads when you install the winch. Using lube on the threads, you'll actually over-stress the bolts if you set them to the specified torque. If you believe in anti-sieze, then lower the torque by about 10%. (I would also consider locktite a "lubricant" if you use locktite on the bolts. )

Oops, if you're asking about bolts to hold the winch mount to the truck, not the winch to the plate, then it all depends on the geometry... A Jeep-style mounting plate would have four or six bolts in almost the same plane as the winch feet, so the loads are very similar. A little overkill never hurts. If the mount is cantilevered, then you need more clamp to hold it, and hence bigger bolts...

Good Luck!

 

[Retired Tanker]

Wow. These are awesome responses! How do I nominate this for a sticky?

(Do we still do stickies?....)

 

[DaveInDenver]

@1stDeuce is the go-to here for sure. I'm not a mechanical engineer (I am however registered P.E., so under no circumstances is my advise to be taken as actionable and you should consult an engineer knowledgeable with your specific situation).

Suffice to say, I thought hydrogen embrittlement had to occur with fairly specific conditions, mainly that the fastener had to be electro-plated and it was thought to be a result of the pre-plating cleaning acids affect on hardened fasteners. It's interesting to me because I used 12.9 bolts on my recovery point and will now be re-evaluating this ASAP!

 

[Hillbilly Heaven]

Dave that was my understanding too that usually occurs with chrome plating.

 

[1stDeuce]

They used to say hydrogen embrittlement was due most often to improper zinc plating of hardened parts. The plating process drives hydrogen into the grain structure, and requires baking fasteners for long periods of time to release the trapped hydrogen, which will otherwise cause inernal stress within the bolt. The harder the part, the more a concern.

It's not an issue at all below about HRC40. Grade 12.9 bolts are well above that... Grade 5/8/8.8/10.9 are below.

We have since come to understand that hydrogen can migrate into a stressed (read torqued to spec) bolt from the environment in some cases, and eventually cause a failure in a fastener that was never plated, or was where a chromate-free coating was used. (Chromate free is a generic way to describe the gray coating used on modern automotive fasteners. Geomet is a poplular brand thereof.)

Also bolts that hard tend to be brittle, and will snap rather than yield if over-stressed. Not an ideal failure mode for a lot of our applications...

 

[DaveInDenver]

Good information. So we should not use 12.9 for anything? The preferred solution would be to use a larger 10.9 if possible. I suppose in my case with the recovery point, since the captured nuts in the frame are what they are, that the marginal theoretical improvement in strength isn't worth the risk of embrittlement.

The point about being more brittle has been tossed around for as long as I can remember with respect to grade 5 vs 8, too. The principle being that as you heat treat to achieve a higher tensile strength the bolt becomes more likely to fracture rather than deform. But what I thought this meant is a fastener as it is hardened just has a decreasing plastic region. I guess you could reduce it to the point of it being practically nonexistent, though?

My thinking absent an anomaly such as embrittlement the grade 5 will reach its ultimate and failed before you even reach the yield region of a grade 8. So while in theory if you exceed the working limit of a bolt the grade 5 will give you a relatively larger buffer to overload before total failure it *will* eventually fail and that will be at a point still lower than the grade 8. IOW, just because a grade 8 might fracture or shear its threads rather than bend it will still always do so at a load higher than the grade 5 will have certainly bent and then failed.

 

[arctic04trd]

FWIW my 8.8 M10s didn't hold up well to the Vr8000-s in an ARB. Ive had maybe 12 pulls, all of which were hard. Fully stuck vehicles. The washers deformed as well as the head of the bolt. The mounting in a Tacoma ARB is feet forward. I can include pics if you would like.

I'm all for the math. Some practical examples don't hurt either. This thread was a good read.

 

[DaveInDenver]

Please do post photos! I'm having trouble picturing how bolt heads can deform in an ARB mount without front face of the bumper bending.

 

[arctic04trd]

Will do. Soon as I get home from work.

The edges of the hex get "squished" as the bolt flexes. The actual bolt appears straight still. I believe it's more to do with side forces not straight pulls.

 

[DaveInDenver]

Are you using a roller fairlead or have you gone to a hawse type?

I ask because I'm still using the old style fairlead that I fitted with Delrin rollers and I wonder if having the fairlead cage, which you have to drill to fit and ends up under two of the winch mounting bolts, helps increase rigidity of the bumper. Two of the bolts have in effect probably a 1/2" (maybe more?) thick washer under them and there's all that extra bracing tying the void in the bumper together.

I haven't seen any fastener issues other than some of the washers tend to deform when they get forced into a slot or oversized hole.

It's always seemed to me kind of a kludge to use those hawse fairleads on ARB bumpers, which weren't intended originally to work that way. So I could absolutely see the front face of them flexing is what I'm saying.

 

[Hillbilly Heaven]

I am one of the of the lucky guys that still has an ARB Classic bumper for the Bronco. I use a Warn M12000 and a roller fairlead. My winch is attached from the bottom with 4 bolts and and bolts in from the front.