View Full Version : Winch anchor for Lefthand Canyon

04-11-2010, 03:10 PM
Below is an email I got from Brian Rasmussen of the Boulder Ranger District.

All you mechanical engineer types, chime in.

-----Original Message-----
From: Brian D Rasmussen
Sent: Apr 11, 2010 10:31 AM
To: w_morgan
Cc: Catherine Luna
Subject: Mike Caskey's winch anchors

Hi Bill,
It been a long time, I hope you are doing well. Cat and I are working with a Forest Service engineer who is interested in finding out about the winch anchor that was used at Jenny Creek Trail. We are still trying to get our winch anchor act together for Lefthand.

It appears that Off-Road Solutions is out of business but we are wondering if Mike Caskey still might be able to fabricate a couple winch anchors. Do you know how we might be able to contact him?
Brian D. Rasmussen
Natural Resources & Recreation Technician
Boulder Ranger District
(303) 541-2525
From: w_morgan
To: Brian D Rasmussen <bdrasmussen@fs.fed.us>
Cc: Catherine Luna <cluna@fs.fed.us>, Rising Sun Land Use

Subject: Re: Mike Caskey's winch anchors
Date: Apr 11, 2010 3:08 PM

Mike now lives and works in Kansas for a Toyota dealer, and I don't think he is doing any fabrication work. However, the winch anchor is pretty straightforward and can be fabricated by any competent welder or 4x4 shop. I would suggest Christo Slee at Slee Offroad in Golden. He does high quality work and is fairly close to you. Altitude Steel can also fabricate this, but may need some pointers on cutting a clevis mount. It is very simple though.

There are a number of ways to build a winch anchor, but as Jenny Creek's seems to have endured the test of time, this is what I recall.

The essence of the winch anchor design is this: if I recall correctly, it starts with a 1/2" thick plate approximately 8" square. To this, a 1" thick clevis mount is welded. It would be recommended to cut a rectangular hole in the 1/2" thick plate, allowing welding on the back AND the front of the clevis mount for strength. Any 4x4 shop will know what a clevis mount is.

Then to anchor it to the TOP of the rock (so that the anchor is in shear rather than tension under use), four RedHead epoxy anchors were used. I do not recall the exact diameter, but essentially a hole is drilled in the rock using the winch anchor as a template, the epoxy fastener is activated and inserted, and driven into the rock. This would present threaded rod (I want to say, 3/4" diameter, but it should be checked for shear force). Before the epoxy sets up, make sure the threaded rods are aligned by loosely fitting the winch anchor to them. When they begin to set up, loosely run the nuts and lockwashers on the threaded rod. Tighten securely the next day or so or when RedHead says the anchor is at full strength.

We put a clevis or "D" ring on the winch point, and spot welded the screw-in part of it to prevent stealing. This is optional as most capable 4x4ers carry their own D rings, but maybe in Lefthand, that might not hold true ;-).

I would guess that the design should be to withstand at least 30,000 lbs. of shear force (more if you want more safety margin, and as far as I know the design above does handle more). That is easier to accomplish than it sounds. I am copying our Rising Sun Land Use mail group and I think there is an engineer or two on the list, who can probably verify this.

I hope that helps, and thanks for contacting me!

Uncle Ben
04-11-2010, 06:25 PM
Good answer Bill!

04-12-2010, 04:48 PM
My vote is for Altitude to do the work. I dropped off a clevis mount for them to do a quick copy of that I bought at High Country. They have the design now and can cut out one in a few minutes. As far as the plate is concerned, thats childs play. Total cost of the two pieces would be around 30 I'm guessing. They quoted me 20 for the clevis mount for future projects.

04-14-2010, 08:16 AM
My only design feedback to the one that ORS made is the holes that went through the plate, for bolts to go down into the rock, were too close together. It was really difficult to bolt that D-ring in there because we kept hitting the bolt heads, and we couldn't put the D-ring in first because then the bolts wouldn't go in.. I guess we ultimately got it done, but at the time I was thinking another half inch of clearance and this would have been done a half hour ago.

04-14-2010, 08:48 AM
So what's needed here? Does somebody need to design it, do the math to prove it holds 30,000lbs, and come up with drawings so it can be built?

I could get material from altitude and build it if needed, pretty cheap stuff from their scrap pile. Also pretty simple welding job that would be bomber if welded on both sides as suggested.

My only question is the strength of the anchor bolts since the ones in the back of the plate would be in tension a certain amount, but I guess that's been proven with the Jenny Creek Anchor.

Does anybody have a picture of the Jenny Creek one?

04-14-2010, 09:51 AM
Does anybody have a picture of the Jenny Creek one?

Actually yes, and an awesome one at that. http://www.risingsun4x4club.org/forum2/showpost.php?p=17624&postcount=44

:bowdown: Treeroot!

04-14-2010, 09:55 AM
Wow, that's sweet!

So you guys want me to just make one or what?

04-14-2010, 10:03 AM
If I understand the loading, two bolts are maybe doing more of the work since they are partially in tension. The only way you can guarantee shear is to be parallel to mounting plane, if you are above or below the anchor, then two of the bolts are loaded in combination tension and shear and two of the bolts are only providing partial shear. Tension is not automatically bad for a properly installed anchor, they are only slightly less strong that way and the load will be generally very close to shear anyway.

The equation to calculate combined shear and tension is:



BTW, 3/4" wedge anchors are not rated to 30ksi even in 6000 psi concrete, so you can't necessarily assume 30,000 lbf for each anchor bolt (unless Red Head told you different for rock compared to concrete). It's about 25,000 lbf or there about, but this would have a decent factor of safety (probably 4x) and so each bolt should provide about 6,000 lbf with 4x margin and so a 24,000 lbf total load is probably justifiable with 4 times factor of safety and 30,000 is probably not dangerous.

I don't like that the shackle is welded in place, this changes the heat treat and does so unevenly across the connection, which assumes double shear on the shackle pin. I understand the need to keep it from getting stolen, though.

I attached a white paper on wedge anchors that might help see what calcs need to be done to certify the anchor.

04-14-2010, 11:09 AM
I don't think a small tack weld on the outer part of the shackle is going to do anything...

we weld lots of hardened stuff and typically the heat treat stays fine within a few milimeters of the weld. that is if it's a quick weld that dosen't allow much heat soak.

so making a few assumptions which are here:

15,000lb winch pull, straight in line with the plate that's bolted.
8" square plate with anchor in the middle
two bolts in line with the actual clevis (like the ones in the jenny creek anchor).
clevis is mounted 2.5" from the plate bottom (centerline of clevis anyways).

and looking at this (if you scroll down a bit it shows a similar setup):

I got this:

the maximum shear stress in each of the bolts is about 23,000psi. that takes into account both the shear and tensile loading of the bolts. Like dave said, they are in tension as well.

So.... looking at the proof stregnth of grade 8 bolts (120,000psi).. and considering we would be using all thread (so I used the minor thread diameter) I got the following:

3/4-10 all thread holds about 40,000lbs
7/8-9 all thread holds about 54,000lbs
1"-8 all thread holds about 70,000lbs

These are all sizes that are available at Mcmaster Carr in all thread and grade 8.

I didn't take into account the stresses induced by tightening the nuts that would be there.


you pick what bolt size you want for what safety factor..

3/4-10 all thread holds about 40,000lbs SF= 1.7
7/8-9 all thread holds about 54,000lbs SF= 2.3
1"-8 all thread holds about 70,000lbs SF= 3

can anybody check my math? That all thread is $60 for a 12" stick of the 3/4-10 stuff and $100 for a 12" stick of the 1-8 stuff BTW.

I looked on the readhead site and that'd be about the right length to use I guess.

04-14-2010, 11:55 AM
I wanted to add that these are pretty rough estimates.... the fact that the anchor has worked well at Jenny Creek speaks volumes.

Also, I'd bet that you could not even cement the all thread into the rock and as long as it was a straight pull the anchor would stay put..

04-14-2010, 12:25 PM
I may have something that will work right now, powdercoated and with 4 holes in it.

It is all 3/4" HRPO, and not only is welded but also drilled and tapped for 3/8-16 SHCS x 2.

04-14-2010, 12:39 PM
There ya go! 4 bolts spaced away from the clevis are even better!

04-16-2010, 05:16 PM
Awesome, I sent Brian and Cat the link to this thread. Should give them all they need.