Understanding Torque Specs and Torque Wrenches
#1
11-13-2018, 10:30 AM
Understanding Torque Specs and Torque Wrenches
I suppose it's 'natural' to assume certain things about torque wrenches and specs because just about everyone I've known, professional or not, has the same basic 'understanding' in regard to torque specs and torque wrenches. However, those 'natural' ideas/assumptions are typically wrong so I thought I'd take a moment or two to pull together some information and pass it along.
First, when looking up torque specs they are almost always displayed as " 18 - 22 lbs/ft " or similar. Almost everyone I've known or talked to regards the numbers as a range to fall into. A few, that should have known better (QC engineers), had even told me years ago that the difference was between new and used fasteners. The point is neither is correct. Here's a simple ten minute video that explains fasteners and torque specs:
Even if the rest of this post makes your eyes roll back into your head the video is worth watching.
Next, there must be a billion websites and videos explaining "How to calibrate Your Torque Wrench" by means of attaching two torque wrenches together by their drive ends, clamping the end of one's handle in a vice and yanking on the other one to see if they 'click' at the same time. This is so wrong. While you may think it's 'close enough' it isn't. If you think 'close enough' is a good idea there's really not much point in using a torque wrench to begin with, is there? :-) ISO Standard 6789 allows for a tolerance of 4 - 6% deviation depending on the type of tool (they currently define twelve classes of torque measuring tools.) Point being torque wrench 'A' could be off 6% and still be considered calibrated and likewise torque wrench 'B' could also be that far off. And then most guys won't have two $500 torque wrenches so one is their 'good wrench' and the other is their $19, 'you know where they go it' wrench. So, even if both fall into the 6% tolerance that adds up to a possible 12%. O.K., so... 12% doesn't sound like much, does it? Let's take a Grade 8 ½" head bolt as an example. Assuming its torque spec is 96 lbs/ft, 12% is 11½ lbs. No big deal, right? Well, when you consider that it's the clamping force that's the big ticket things change a bit. That ½" bolt has a clamping force of 12,771 lbs at that torque. Being 12% off could mean a reduction of around 1,533 lbs. Kind of a game changer, eh? (http://tinyurl.com/ybrzla4x ) Using a 3/8" rod bolt you could lose/gain around 948 lbs., that would probably have some sort of effect on bearing crush, don't you think?
Which brings us to how to check calibration. The same ISO Standard calls for calibration every 5,000 cycles or every 12 months, whichever comes first. Not many people are going to follow that, first because they never have in the past, it costs too much, nobody nearby does it, etc. Here's a reasonable way to check one: https://www.wikihow.com/Calibrate-a-Torque-Wrench . Keep a couple thoughts in mind: whatever weight is used should be checked to be its actual weight i.e. just because a dumbbell has "10 lbs." cast on it doesn't make it accurate. Also, the wrench should be checked at 20%, 60% and 100% of its maximum rating. Checking a 250 lbs/ft wrench with a 20 lb. barbell weight and calling it good is pretty sketchy.
I know this has been a little long-winded but, as with so many things in life, what we sometimes take for granted has a lot more involved than we ever suspected. I just wanted to help the guys that want to rebuild their engine, etc., I always like seeing someone willing to learn something new in their hobby.
First, when looking up torque specs they are almost always displayed as " 18 - 22 lbs/ft " or similar. Almost everyone I've known or talked to regards the numbers as a range to fall into. A few, that should have known better (QC engineers), had even told me years ago that the difference was between new and used fasteners. The point is neither is correct. Here's a simple ten minute video that explains fasteners and torque specs:
Next, there must be a billion websites and videos explaining "How to calibrate Your Torque Wrench" by means of attaching two torque wrenches together by their drive ends, clamping the end of one's handle in a vice and yanking on the other one to see if they 'click' at the same time. This is so wrong. While you may think it's 'close enough' it isn't. If you think 'close enough' is a good idea there's really not much point in using a torque wrench to begin with, is there? :-) ISO Standard 6789 allows for a tolerance of 4 - 6% deviation depending on the type of tool (they currently define twelve classes of torque measuring tools.) Point being torque wrench 'A' could be off 6% and still be considered calibrated and likewise torque wrench 'B' could also be that far off. And then most guys won't have two $500 torque wrenches so one is their 'good wrench' and the other is their $19, 'you know where they go it' wrench. So, even if both fall into the 6% tolerance that adds up to a possible 12%. O.K., so... 12% doesn't sound like much, does it? Let's take a Grade 8 ½" head bolt as an example. Assuming its torque spec is 96 lbs/ft, 12% is 11½ lbs. No big deal, right? Well, when you consider that it's the clamping force that's the big ticket things change a bit. That ½" bolt has a clamping force of 12,771 lbs at that torque. Being 12% off could mean a reduction of around 1,533 lbs. Kind of a game changer, eh? (http://tinyurl.com/ybrzla4x ) Using a 3/8" rod bolt you could lose/gain around 948 lbs., that would probably have some sort of effect on bearing crush, don't you think?
Which brings us to how to check calibration. The same ISO Standard calls for calibration every 5,000 cycles or every 12 months, whichever comes first. Not many people are going to follow that, first because they never have in the past, it costs too much, nobody nearby does it, etc. Here's a reasonable way to check one: https://www.wikihow.com/Calibrate-a-Torque-Wrench . Keep a couple thoughts in mind: whatever weight is used should be checked to be its actual weight i.e. just because a dumbbell has "10 lbs." cast on it doesn't make it accurate. Also, the wrench should be checked at 20%, 60% and 100% of its maximum rating. Checking a 250 lbs/ft wrench with a 20 lb. barbell weight and calling it good is pretty sketchy.
I know this has been a little long-winded but, as with so many things in life, what we sometimes take for granted has a lot more involved than we ever suspected. I just wanted to help the guys that want to rebuild their engine, etc., I always like seeing someone willing to learn something new in their hobby.
#2
11-13-2018, 12:31 PM
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#5
11-13-2018, 04:49 PM
Smitty - It kind of comes with the territory. Years ago approaching and/or exceeding 1 hp/cubic inch was considered a pipe dream. Today engines that do are pretty common. But it requires new tech and procedures to do it reliably. Aluminum heads and blocks stretch and contract like crazy compared to cast iron. As a result new gaskets and fasteners were designed and procedures changed to accommodate them. This page at FelPro can explain it better than I can: https://www.felpro.com/technical/tec...t-y-bolts.html If you think torque-to-yield is a PITA try ARP fasteners. If they aren't torque-to-yield they want you to torque to/by spec, loosen the fastener, re-torque, loosen the fastener again and perform final torque. They do that because testing has shown that the micro-burrs on the threads are worn smooth by the process yielding a more accurate final torque/clamping force.
#6
11-13-2018, 04:58 PM
Ross - when in a situation like that I use a little 3-in-! on the threads. It's better than dry but not so slippery as to veer too far away from spec. I look at it this way - there weren't any really slippery oils available back then. Parts were made made well in advance of assembly (nobody had heard of JIT - 'Just in Time' - inventories yet) so they probably had some sort of oil on them for storage. Maybe not exactly a lubricant but also not dry, pretty much the same attributes of 3-in-1. :-) I can't imagine the guys on those old assembly lines taking the time to squirt some fresh oil on the threads before assembly.
#7
11-14-2018, 11:39 AM
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#9
11-15-2018, 11:00 AM
Skimmed through the video a couple times but didn't hear an explanation for the torque spec range usually supplied to fasteners e.g. "65 to 90 ft/lbs" mentioned in the first post.
The '64 Ford Truck Shop manual specifies that wheel lug nuts are for lubricated bolt threads. The '64 Operator's manual, on the other hand, sez dry threads At some point this generally changed to "you're all gonna die" with regard to lubricated lug nut threads in most literature and auto websites etc. Probably a few rounds of unregulated Air-tools, idiots, + oiled threads and warranty repairs on their dime is the cause of that. I use a small amount of anti-sieze, juuust enough to keep them from screeching, and the lowest torque @ 65 ft/lbs. Regardless of what you do it's important to periodically check wheel nut tightness. Just do it, it's only a few minutes.
One thing that concerns me, when fasteners and parts were made with good Henry steel (vanadium, high nickel content, etc) there was a lot of headroom on how much abuse a bolt or component could take. With parts today often made of golly knows what, sold by the pound (from golly knows where) they are not likely to stand up to the overtorquing that was common, no way now how. "Grade 8" bolts sold by the pound at the hardware store? Suuure.
A torque spec range is useful when a castellated nut & cotter pin is required. Typically a castellation and the bolt hole will line up somewhere along the min. and max. torque. If not, try a different nut, or, an additional thin shim or washer under the nut will get things lined up within the spec torque range.
The '64 Ford Truck Shop manual specifies that wheel lug nuts are for lubricated bolt threads. The '64 Operator's manual, on the other hand, sez dry threads At some point this generally changed to "you're all gonna die" with regard to lubricated lug nut threads in most literature and auto websites etc. Probably a few rounds of unregulated Air-tools, idiots, + oiled threads and warranty repairs on their dime is the cause of that. I use a small amount of anti-sieze, juuust enough to keep them from screeching, and the lowest torque @ 65 ft/lbs. Regardless of what you do it's important to periodically check wheel nut tightness. Just do it, it's only a few minutes.
One thing that concerns me, when fasteners and parts were made with good Henry steel (vanadium, high nickel content, etc) there was a lot of headroom on how much abuse a bolt or component could take. With parts today often made of golly knows what, sold by the pound (from golly knows where) they are not likely to stand up to the overtorquing that was common, no way now how. "Grade 8" bolts sold by the pound at the hardware store? Suuure.
A torque spec range is useful when a castellated nut & cotter pin is required. Typically a castellation and the bolt hole will line up somewhere along the min. and max. torque. If not, try a different nut, or, an additional thin shim or washer under the nut will get things lined up within the spec torque range.
#10
11-15-2018, 03:00 PM
Notice the chart in the video? Look at the column headings and you'll see the difference is lubricated vs. dry - not a range I suppose I should have covered that.
We all know the adage 'you get what you pay for' has a certain amount of truth to it. Sometimes you pay more and get less. That's when you seek out information from credible sources and not make a decision based on price alone
We all know the adage 'you get what you pay for' has a certain amount of truth to it. Sometimes you pay more and get less. That's when you seek out information from credible sources and not make a decision based on price alone
#11
11-16-2018, 07:30 AM
A couple of questions that need more answer.
Torque to Yield is by far more accurate. The torque value is to just preload the joint and make sure it is "stiff" with no gaps or what we call a "smile" in the joint. Then the degree of rotation is where we put the load or stretch into the bolt. The turn part is so accurate, because if you know the TPI, you can figure out exactly how much longer the bolt/stud has become. For example, if you have a stiff flange joint with a 1"-8TPI that needs to be tightened and we tighten the nut 1 revolution it will get 1/8" longer. Friction plays NO factor in the tightening at this point. It must be 1/8" longer. You should consider that every Angle of Turn bolt is a 1 time use bolt/stud. They very likely have been tightened to a value that is very close to yield of the fastener. Toss them and get new ones.
The fastener will lose 12-18% of it's load over 30 days. The is cause by embedment into the surface by the niy and the head of the bolt. The threads will deform to each other also contributing to this loss. The engineers have calculated this all in during the design phase.
The Torque Range is usually Plus/Minus 10% of the desired spec. A 100 Foot Lb. spec us gonna be indicated as 90-110 Ft. Lbs. The engineer typically (not always depending on application) uses a value of 50% of the material yield strength to determine the torque range. So, if a bolt/stud has a yield strength of 100,000 Lbs., the desired load will be 50,000 Lbs. That load is then converted to a torque spec. Friction is a HUGE unknown in all of this so pay attention to the lube specified and make sure you lube the threads and the contact surface of the but or bolt head. There is actually more friction between the face of the nut/bolt head than the threads! Different lubes will also change the torque spec quite a bit. Copper or Nickel Anti-Sieze will be a good everyday lube for you to use. Moly Paste is even more slippery and will reduce the torque value even more.
The grade of bolt material is tightly controlled, I agree that offshore nuts and bolts can be dicey and not made from the proper material or not made to the spec. If you are needing mission critical nuts and bolts, head to Fastenal, McMaster Carr or Granger. Maybe there is a hardware store (not the Ace Hardware) but a fastener supply or bearing house.
Do not substitute stainless for mission critical bolting. Mounting fenders, that just fine. Brake calipers or motor mounts, that's no good.
Always turn your torque wrench back to it's lowest setting when putting it back in the tool box.
Alway use the grip of the wrench to pull/push to tighten.
Do not use a torque wrench to 100% of it's range. The normal commercial torque should be used in it's 20% to 80% of it's range. Need to go lower or higher? get the appropriate 20-80% wrench. ie. Your 250 Ft Lb torque wrench should not be used below 50 Ft. Lbs. nor above 200 Ft. Lbs.
We use 3 passes for most tightening and than make 1 more pass to make sure all all up to spec. Belts and suspenders!
Too much info!! There is a lot of deign work and thought that goes into the design of a joint. Use the proper tools and follow the manufacturer info and you will be good to go.
Torque to Yield is by far more accurate. The torque value is to just preload the joint and make sure it is "stiff" with no gaps or what we call a "smile" in the joint. Then the degree of rotation is where we put the load or stretch into the bolt. The turn part is so accurate, because if you know the TPI, you can figure out exactly how much longer the bolt/stud has become. For example, if you have a stiff flange joint with a 1"-8TPI that needs to be tightened and we tighten the nut 1 revolution it will get 1/8" longer. Friction plays NO factor in the tightening at this point. It must be 1/8" longer. You should consider that every Angle of Turn bolt is a 1 time use bolt/stud. They very likely have been tightened to a value that is very close to yield of the fastener. Toss them and get new ones.
The fastener will lose 12-18% of it's load over 30 days. The is cause by embedment into the surface by the niy and the head of the bolt. The threads will deform to each other also contributing to this loss. The engineers have calculated this all in during the design phase.
The Torque Range is usually Plus/Minus 10% of the desired spec. A 100 Foot Lb. spec us gonna be indicated as 90-110 Ft. Lbs. The engineer typically (not always depending on application) uses a value of 50% of the material yield strength to determine the torque range. So, if a bolt/stud has a yield strength of 100,000 Lbs., the desired load will be 50,000 Lbs. That load is then converted to a torque spec. Friction is a HUGE unknown in all of this so pay attention to the lube specified and make sure you lube the threads and the contact surface of the but or bolt head. There is actually more friction between the face of the nut/bolt head than the threads! Different lubes will also change the torque spec quite a bit. Copper or Nickel Anti-Sieze will be a good everyday lube for you to use. Moly Paste is even more slippery and will reduce the torque value even more.
The grade of bolt material is tightly controlled, I agree that offshore nuts and bolts can be dicey and not made from the proper material or not made to the spec. If you are needing mission critical nuts and bolts, head to Fastenal, McMaster Carr or Granger. Maybe there is a hardware store (not the Ace Hardware) but a fastener supply or bearing house.
Do not substitute stainless for mission critical bolting. Mounting fenders, that just fine. Brake calipers or motor mounts, that's no good.
Always turn your torque wrench back to it's lowest setting when putting it back in the tool box.
Alway use the grip of the wrench to pull/push to tighten.
Do not use a torque wrench to 100% of it's range. The normal commercial torque should be used in it's 20% to 80% of it's range. Need to go lower or higher? get the appropriate 20-80% wrench. ie. Your 250 Ft Lb torque wrench should not be used below 50 Ft. Lbs. nor above 200 Ft. Lbs.
We use 3 passes for most tightening and than make 1 more pass to make sure all all up to spec. Belts and suspenders!
Too much info!! There is a lot of deign work and thought that goes into the design of a joint. Use the proper tools and follow the manufacturer info and you will be good to go.
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