Bolt torque suggestions

Embarrassing to ask since I've placed a fair number of bolts, but a car guy asked me what I torqued my bolts to. I've used a 6" crescent wrench to tighten down bolts. I get to "it's PRETTY hard to turn" and called it good. I looked at a few manufacturer's websites, but didn't see an actual number. (Apparently the 'Benz mechanic was trying to tell me was that all their specs detail what the torque is supposed to be for all applications.) What's proper for expansion bolts that we use, and is a 6" wrench getting me in the standard range? Any thoughts?

Here's a listing for two popular styles of bolts in both carbon and stainless steel from Powers.com which are viewable at their website in PDF form.

Listed below are the maximum recommended torques (in ft-lbs) for two popular sizes of the Power-Bolt as well as the Power-Stud.

Carbon Steel Powers Bolt 3/8" - 20-25 (ft-lbs of torque)
Carbon Steel Powers Bolt 1/2" - 30-35 (ft-lbs of torque)

Stainless Steel Powers Bolt 3/8" - 10-12 (ft-lbs of torque)
Stainless Steel Powers Bolt 1/2" - 20-25 (ft-lbs of torque)

Carbon Steel Power-Stud 3/8" - 28 (ft-lbs of torque)
Carbon Steel Power-Stud 1/2" - 60 (ft-lbs of torque)

Stainless Steel Power-Stud 3/8" - 28 (ft-lbs of torque)
Stainless Steel Power-Stud 1/2" - 60 (ft-lbs of torque)

As you can see, the recommended torque values are lower for Stainless Steel in the Powers-Bolt, and interestingly enough, they are exactly the same for the Power-Stud.

Also see this discussion on bolt torque.

Don't we have never ending bolt torque discussions with the Wasatch/LCC guys? Oh, wait- thats torqued about bolts.

rob bauer wrote:What's proper for expansion bolts that we use, and is a 6" wrench getting me in the standard range? Any thoughts?

I have no doubt I could twist the head off a bolt with a 6" wrench and have had friends actually do it several times.

Over torque, in my opinion, is much worse than under torque.

If you don't have a torque wrench, than, at the very least, follow the installation guidelines for the bolt (number of turns, etc). Especially with stainless, its best to err on the side of less torque, than more.

Note that Powers dumped their installation torque down to 12 foot-pounds for the 3/8" powerbolts.

I found a really short (like 3" or so) 3/8" drive socket wrench that is fast and makes it hard to over torque, since the handle is so short.

Brian in SLC wrote: I have no doubt I could twist the head off a bolt with a 6" wrench and have had friends actually do it several times.

In the other thread, Greg Hand reported that it was "all he could do" to attain 20 ft-lbs. of torque with a 6" wrench.

Casey Bernal reported it was "borderline painful" to get 25 ft-lbs. of torque with an 8" wrench.

As long as the torque spec. for your bolts is in the 20-25 ft-lbs. range, I wouldn't worry about overtorquing with a 6" to 8" wrench.

Agreed, that it is important not to over-torque; however, under-torquing will result in the hanger spinning after a few falls -- so it's best to try to get it in the proper torque range. Additionally, Powers specs are based on the bolts in that range...in other words the strength for pullout (probably not shear) may be less if the bolt is under the specified torque range.

Tony Calderone wrote:Important Note: There is a very good reason why the manufacturer does not have a minimum torque specification. The only reason for tightening anchors at all is to ensure the grabbing mechanism is engaged. This is easily achieved with very little torque. If you want additional insurance the mechanism will remain engaged over time I recommend using a locknut and/or lockwasher. The compression ring on Power-Bolts acts in a similar capacity.

Powers lists maximum tightening torque for their Power-Bolts, but other manufacturers list recommended torque. The instructions for the Hilti Kwik Bolt 3 say "tighten the nut to the recommended installation torque". Thunderstud wedge anchors and Fixe Triplex bolts also list recommended torque in their installation instructions.

The general consensus amongst engineers is that bolt torque is as reliable a technique for preloading a bolt as the rhythm method is for preventing pregnancy. It’s surprising that even car mechanics struggle with the concept of preload. I tried to explain preload to a mechanic once, and he looked at me like I was from Mars. It was sort of like trying to explain evolution to a fundamentalist Christian.

My Mechanical Engineering Reference Manual states that “the standard torque wrench does not provide precise, reliable preloads, since the fraction of the torque going into bolt tension is variable.” Friction is especially unpredictable and variable in climbing anchors, due to things like dirty holes; rock dust and fragments that get into the threads, etc.

Preload is essentially the amount of clamping force in a bolted joint. Torque is simply a technique used to indirectly measure preload, and is accurate to within 40% (that is plus or minus 40%), which is a pretty sloppy range. This means that using a torque wrench to measure preload on a climbing anchor is pretty damn unreliable.

The “operator feel” method is prone to a 35% error and torque wrench to 25% (according to the roymech website), and the other (more accurate) techniques are impractical for installing climbing anchors.

Check out the links below for more info:

zerofast.com/torque.htm

steelstructures.com/SBH/SBH…

roymech.co.uk/Useful_Tables…

I’ve broken a 3/8 Powers bolt using a 6 inch wrench without pain, let alone discomfort, and I wasn’t trying to break it. Fortunately, I’ve since learned to use a lot less torque (using the old "operator feel" method).

I’ve also adopted the practice of using ½ inch plated carbon steel Powers bolts or 12mm stainless, instead of 3/8 inch. These sizes are much more forgiving, as it becomes very difficult to stretch the bolt beyond the recommended preload (“over-torqueing”) using a 6 inch wrench handle. If you’ve ever hammered in a ½ inch bolt, you quickly realize that it’s probably got the pull out strength of the most bomber piton placement, even before tightening.

In my opinion, don’t use skinny 3/8 bolts, and you won’t have any worries about breaking bolts or placing time bombs.

Nate Adams wrote:I’ve broken a 3/8 Powers bolt using a 6 inch wrench without pain, let alone discomfort, and I wasn’t trying to break it. ... In my opinion, don’t use skinny 3/8 bolts, and you won’t have any worries about breaking bolts or placing time bombs.

I would say don't use 3/8" stainless-steel Powers Power-Bolts; these are the ones that are prone to breaking by over torquing. They have a maximum installation torque of 10-12 ft-lbs.

Other 3/8" bolts (Hilti Kwik Bolt 3, etc.), with a recommended installation torque of 20-30 ft-lbs., are fine as climbing anchors in hard rock, and are not prone to breaking by over-torquing with a 6" wrench.

You can always use an actual torque wrench, too. I used to shlep one around with a keeper cord and everything.

Brian, what was that about lowering the torque recommendations to 12 ft/lbs for 3/8" bolts?

Did anyone else see that?

tenesmus wrote:Brian, what was that about lowering the torque recommendations to 12 ft/lbs for 3/8" bolts? Did anyone else see that?

Powers used to recommend 20 ft-lbs. They've lowered it to 10-12 ft-lbs in the last year or two.

Makes me wonder why, but, its an awefully low torque compared to the past, and, other fasteners like the Hilti, Fixe, Raumer.

Number of turns is a good rule of thumb.

I'd rather have to re-tighten a loose bolt, than over torque one.

Just a note on what Ron said - I know of many cases of people breaking 3/8" 5-pieces by overtorquing. I've snapped dozens while unscrewing old ones which were probably overtorqued to begin with. But every single snapped bolt I have heard about is carbon steel, not stainless steel - which is probably due to the fact that no one uses stainless, but might be because stainless stretches first instead of snapping? Anyone know of a case of someone snapping a stainless?

"5-piece Rawls", whether old and corroded or brand new installations, tend to seize in the hole. So, when you try to remove the bolt, the bolt isn't actually turning, and you end up snapping the bolt off near the mouth of the hole.

All steel stretches when a load is applied. Stainless steels tend to have a lower yield strength than carbon steels, which is why stainless steel appears to be more brittle.

Manufacturers calculate recommended installation torque based on yield strength (which is material dependend), size and friction assumptions (which is also material dependent - stainless steel has different coefficients of friction than plated carbon steels). That's why stainless steel bolts have lower recommended torque values.

Friction determined empirically under laboratory conditions is NOT going to be the same as the friction in your bolt installation on your next project. Which is, again, why torque is a wildly inaccurate method of measuring preload.

Keep in mind that a 3/8 five-piece Rawl uses a 5/16 diameter bolt, whereas a 3/8 Hilti Kwik Bolt 3 uses a 3/8 diameter bolt, so the Hilti is stronger at the shank (though not necessarily at the taper).

All this discussion about torque evades the issue of corrosion and longevity. A correctly preloaded 3/8 bolt will not last as long as a correctly preloaded 1/2 inch bolt, which is another reason to use bigger hardware. I know most of you use pretty sweet Bosch and Hilti drills, which can handle 1/2 inch holes, so why not install fatties and do everyone a favor?

Nate Adams wrote:All steel stretches when a load is applied. Stainless steels tend to have a lower yield strength than carbon steels, which is why stainless steel appears to be more brittle.

Methinks you may have meant "ductile" instead of "brittle"?

Stainless bolts, especially 300 series, are prone to galling of the threads. Common to apply some type of lube for some installations. A metallurgist friend suggested body oil even to lube the threads slightly to prevent galling, which might ruin a placement. I think if you've had issues (and I know someone on this thread that has) with galling (which ruins a stud placement), you might think to maybe put a little lube on the stud prior to putting the nut on. Probably a joke here...but...

I used to think pre-mushing the cones of my Powerbolts made them easier to place and easier to tighten (especially one handed on lead). But, for stainless, this ends up ruining the bolt as the metal is so soft and ductile (compared to grade 5 or 2) that the cone just sticks to the threads and becomes a spinner. Drill bit size (and wear) becomes more of an issue with installing stainless too. I try to watch my wear a bit more when installing stainless. Grade 5 powerbolts aren't near as fickle.

Nate Adams wrote:A correctly preloaded 3/8 bolt will not last as long as a correctly preloaded 1/2 inch bolt, which is another reason to use bigger hardware. I know most of you use pretty sweet Bosch and Hilti drills, which can handle 1/2 inch holes, so why not install fatties and do everyone a favor?

Darn things tend to be pretty huge, and, a bunch of them doesn't fit in a bag near as well (and heavy). 1/2" seems to be over kill for most stuff. Also, for 1/2" studs, I've found it hard to find prepainted hangers that will accept a 1/2" diameter stud (or 12mm). The Powers 1/2" stainless bolts are very cost prohibitive too. But, at least they can be disassembled to fit a 10mm hole on most hangers.

1/2" is a darn big hole too. Spare drill bits take up space and weight as well.

But, yeah, not that much of an issue, drilling wise, in most granite type rock and the like.

I did mean brittle, in the sense that 304 or 316 SS has a lower yield point than Grade 5 carbon. If you stress a 3/8 stainless bolt to the yield point, the same size Grade 5 bolt has the capacity to take additional load before it begins to yield. An analogy would be a thin rubber band (SS) and a thick rubber band (Grade 5) - under a given load, the thin rubber band breaks before the thick one. I don't mean to sound patronizing... feel free to tell me to piss off if I do :)

Body oil will help prevent galling, even better are anti-sieze/lubricating compounds made specifically for stainless steels, such as Markal EZ-Break, which is copper based and comes in dry stick form. You can get this through Grainger, or the local hardware store might stock equivalents. Oils and lubes change friction coeffiecients, so toss recommended torque values out the window.....do I sound like a broken record on this subject?

Yeah, you do get a better workout lugging around a bag of fatties and 1/2" bits :) I certainly can't afford 1/2" SS bolts; I found a source for 1/2" Powers (electroplated) for a buck a pop; you can find plated hangers for a buck - you may need to buy in bulk for these kinds of deals. Retail climbing shops tend to screw you on price for bolts and hangers. ASCA is good about providing 12mm SS hardware, even bits, for replacement projects.

Oh, don't forget to match materials. Stainless hangers and electroplated bolts do not mix - galvanic corrosion is not your friend.

We do a lot of hand drilling in granite, and we don't want the bolts to corrode, and we want them replaceable. All of which add up to stainless short 3/8" 5-piece. A lot of people argue for stud-bolt type of bolts in stainless, but they can't be replaced in the same hole in the future.

However - we strongly recommend 12mm or 1/2" bolts (stainless of course) for any replacement work where we can power drill (and you can hand drill those in softer rock like medium-quality rock at Red Rocks). And definitely 1/2" for bolts likely to see heavy use (dogging, belay stations, etc).

Brian in SLC wrote: What's proper for expansion bolts that we use, and is a 6" wrench getting me in the standard range? Any thoughts? I have no doubt I could twist the head off a bolt with a 6" wrench and have had friends actually do it several times. Over torque, in my opinion, is much worse than under torque. If you don't have a torque wrench, than, at the very least, follow the installation guidelines for the bolt (number of turns, etc). Especially with stainless, its best to err on the side of less torque, than more. Note that Powers dumped their installation torque down to 12 foot-pounds for the 3/8" powerbolts. I found a really short (like 3" or so) 3/8" drive socket wrench that is fast and makes it hard to over torque, since the handle is so short.

What was powers recommended torque until then?

As the OP, I’ll end this by saying I got a torque wrench shortly afterwards and still use it when I put in new routes.  AND, now this is all covered in the Hardware Forum.

Stainless an plated bolts From the same company also have different specs.  

Fixe gives torque specs on all the stuff they sell on their site.

All this X size wrench or go till it hurts talk is BS how strong are You?  Are your hands used to turning wrenches like mine?

If you are not gonna use a torque wrench at least borrow one or whatever and torque a few bolts then check em with the box wrench or whatever you use so you have some idea what the spec feels like to YOU.  (Still usually not accurate but as a mechanic I can get pretty close)