Jump to content

Bolt torque


Recommended Posts

Is there a recommended torque that the various bolts should be done up to when building the frame, securing the rails and so on?

I ask only because I've recently bought a micro-torque wrench capable of doing 1-6 Nm and, well you know, its nice to use tools if you have them ūüėĄ

  • Like 1
Link to comment
Share on other sites

This topic came up on a recent Steve Builds live stream. A bunch of people, myself included, did some research on screw torque as it applies to Tee Slot extrusions, and after a good bit of searching, we weren't able to find any T-slot extrusion specific torque settings. Which means you might have to apply some common sense logic.

You could start with a standard torque chart but bear in mind that the extrusion flexes and the standard torque values felt about 30-40% higher than needed to Steve when he had set his torque driver to the recommended torque.

It's important to note that what's really required to hold a screw is that enough tension applied to the screw so that it doesn't back out when subjected to vibration and flex in the system. That tension is achieved in various ways. The screw can elongate or stretch, you can install a split washer (which is a spring with one coil) and in the case of extrusions, the extrusion itself flexes and since we can't easily measure that tension... We can, however, measure the rotational force applied to the fastener when tightening and that's why we use torque. So, what's the correct torque? Enough that there is adequate tension between the screw head and thread to prevent it from coming undone.

I know this all sounds very "shoot from the hip" but when you have no published data in hand... that's, unfortunately, what you're left with. Here's a torque chart and if you feel that the recommended torque is excessive, which it probably will be, then back it off a bit.

In the end, torquing all the screws to achieve the same torque (tension) will, at the very least, ensure uniformity which goes a good way towards minimizing any potential failures down the road.

Hope this helps.

  • Like 4
Link to comment
Share on other sites

  Bolt torques in blind hole applications are a pain to nail down as you found out. I didn't see the Steve Builds stream but here is my thoughts on bolt torques. 

  Allot of the papers written on the topic are based on very large bolts where the elongation can be measured. The best example is the bolts on connecting rods or head studs in an engine. The bolt is literally stretched like a spring and that is what keeps it tight. They are all similar materials and you can actually measure the bolts stretch which lets you negate any other variables like, lubricant, washers, rough or smooth finish, head size (Blah Blah Blah).

 At some point someone tried to take test rig data an distill it into those very confusing charts. At my old job we tried to calculate torques and the system fell very short. Heck my old boss even had all of us blind torque bolts with a beam style wrench by feel. It was quite comical and the friction difference between several head styles, lubricants and surface finish was dramatic. 

 Remember you are always at the mercy of the material the head is landing on. Mountain bikes suffer from this with lots of Steel and Ti bolts and small aluminum counter bores. You will never get a 3mm or 4mm bolt tight enough to make it stretch much when terminating on a softer material. The head just buries itself deeper and deeper especially with fine threads. So we use things like spring washers, Loctite, NyLock etc. Most Allen keys also limit the maximum torque you can apply.

If you want repeatability a torque wrench is great. You could experiment with several settings to hone in on what you think is correct. Many machinists use torque wrenches on fixtures to make sure its the same scenario each part they run. 

Some tables can give you a ball park max a bolt can with stand in ideal conditions.

Bolt toques can be very helpful on plastic threads and very small fasteners. But again some destructive testing is required. 

But the biggest things to look out for is 1) Does it come loose when subjected to vibration 2) Does the head deform the base material 3)Are you damaging the thread at high torques? 4) Think of what the test data was generated for, Engines, Bridges, Sky scrapers, Heavy equipment or some other large commercial industry. 

      

 

 

  

  • Like 5
Link to comment
Share on other sites

I deal with bolt torque at work as well!   The chart above only goes down to 5mm Allen bolts, with 4mm and 3mm left blank.  Probably would only be 1-3 Nm, as the 5mm is at  5.5Nm and the 6mm at 9.5Nm, similar to 6mm stuff I deal with at work which varies between 8.0Nm to 11Nm.

But as mentioned, it doesn't matter much if the bolt is squishing down into plastic anyway!  Pan head bolts and wide washers only go so far and if you get the enclosure nice and warm, squishy squishy!

So, I've been installing CNC aluminum parts in certain key structural areas, especially the TAP assembly and the X carriage.  Perhaps a few other locations are in progress as well. This allows one to get proper bolt stretch and part clamping.

  • Like 2
Link to comment
Share on other sites

Ah, but that part of the chart refers to class 12.9 hex head cap screws, which are very much stronger than our Voron low-grade socket head, or "allen" bolts.

As a point of reference, the low-grade 6mm bolts I know well at work are usually tightened to 8Nm and the higher grade ones  11 or 15Nm.   This chart indicates 17.5Nm "dry" for a class 12.9 6mm hex head cap screw, which I have not yet encountered.  

If I were to tighten the low-grade 6mm bolts at work to 17.5Nm, they would likely break off or strip out and I would have some explaining to do...

  • Like 3
Link to comment
Share on other sites

You need to put enough "gronk" on it that it won't back out nor strip. 

Now THAT is some technical stuff right there!

I Don't really have time to respond like others have here but some of the considerations as others have mentioned is the grade of fastener along with thread engagement. Usually you will see 75% engagement or more.  " The Machinery's Handbook" specifies thread engagement if you want to get more technical.  My book is in my shop at the moment, but could get the data later. 

Most fasteners are considered dry. ANY sort of lubricant significantly changes the specs and sends you out into unknown territory.

  • Like 1
Link to comment
Share on other sites

On 3/14/2024 at 6:27 AM, claudermilk said:

ugga-duggas

56GL10_AS01.jpg.843544c0a7b06853cbb7854c17f0a78e.jpg

One of those ugga duggas? I might need to go re-torque mine. ūüėÖ

 

I just went by feel, the bolts are tiny and with aluminum involved...

Edited by zav3nd
  • Like 1
  • Haha 4
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

√ó
√ó
  • Create New...