Torque Values (Rule of thumb?)
Torque Values (Rule of thumb?)
(OP)
Here are the 2 bolted joint scenarios I'm looking at.
1. I have a 1"-14 bolt with a maximum service loading of 8,880lbf. The bolts are black oxide coated so I'm using a k factor of .25 - .21. My calculations show that I need torque values ranging from 714 ftlbs to 850 ftlbs to obtain the necessary clamp to resist slipping. I am recomending torque of 775 ftlbs to 825ftlbs to obtain necessary clamp. I have clamp loads calculated at 3,543 lbf to 3,771 lbf respectively.
2. I have a 7/8" bolt with a maximum service loading of 5,550lbf. Everything is same with coating. I'm showing a need of 450 ftlbs to 500 ftlbs with clamp loads calculated at 2,057 lbf and 2,286 lbf respectively.
I have been questioned if these torque values are to high for the grade 8 nuts and bolts we are using. From what I can see, I'm staying well below proof loads of both sizes.
So here is my question: Is there a rule of thumb I should consider for 1" and 7/8" bolts? A fellow engineer here as stated that I might consider dropping the 1" bolt to 500 ftlbs and the 7/8" to 400 ftls. I'm concerned that I may not get a sufficient clamp.
Any thoughts?
1. I have a 1"-14 bolt with a maximum service loading of 8,880lbf. The bolts are black oxide coated so I'm using a k factor of .25 - .21. My calculations show that I need torque values ranging from 714 ftlbs to 850 ftlbs to obtain the necessary clamp to resist slipping. I am recomending torque of 775 ftlbs to 825ftlbs to obtain necessary clamp. I have clamp loads calculated at 3,543 lbf to 3,771 lbf respectively.
2. I have a 7/8" bolt with a maximum service loading of 5,550lbf. Everything is same with coating. I'm showing a need of 450 ftlbs to 500 ftlbs with clamp loads calculated at 2,057 lbf and 2,286 lbf respectively.
I have been questioned if these torque values are to high for the grade 8 nuts and bolts we are using. From what I can see, I'm staying well below proof loads of both sizes.
So here is my question: Is there a rule of thumb I should consider for 1" and 7/8" bolts? A fellow engineer here as stated that I might consider dropping the 1" bolt to 500 ftlbs and the 7/8" to 400 ftls. I'm concerned that I may not get a sufficient clamp.
Any thoughts?
Cabbages, knickers, It hasn't got A BEAK!





RE: Torque Values (Rule of thumb?)
For some reference documents, try NASA RP 1228 Fastener Design Manual, RCSC's Guide to Design Criteria for Bolted and Riveted Joints, and MIL-HDBK-60 Threaded Fasteners - Tightening to Proper Tension.
RE: Torque Values (Rule of thumb?)
Any bolt should be preloaded to suit the application not to what somebody thinks and the general rule of thumb where I am working is:- for a joint not intended to be taken apart at anytime it would be preloaded to 90% of proof stress and 70% proof stress for joints that need to be dismantled.
Try this site it may help;-
ht
desertfox
RE: Torque Values (Rule of thumb?)
Thanks to both of you. I feel better now leaving my torque values as they are. My colleague was worried that I would snap the head off the bolts, but I am relatively low in stress so I doubt that is a problem. Well, as long as the supplier truly supplies grade 8...
Cabbages, knickers, It hasn't got A BEAK!
RE: Torque Values (Rule of thumb?)
Here's another site that might be of interest:
http://www.wellheadservices.net/Technical.htm
RE: Torque Values (Rule of thumb?)
Structural people use torque values less and less these days.
RE: Torque Values (Rule of thumb?)
Off the top of my head, an A325 [similar to a Grade 5] requires 28 kips of tension to be considered 'fully pretensioned'.
The problem with torque is that sliding friction a HUGE element in the equasion to mathematically correlate torque to tension/stress. Smoothness of the male & female threads, and the type of lube used make any calculated torque unreliable. For a general torque on 1"-8tpi high-strength bolts/stude, we use 370 ft-lbs.
To go significantly higher than that, like you are, I would either run a 5-bolt sample through a 'Skidmore', or use Direct Tension Load Indicating washers to develop a reasonable torque value for your exact bolts.
RE: Torque Values (Rule of thumb?)
I have found that there are multiple ways to calculate clamp loads and multiple "k" factors that can be used. So what I have done is develop a mathcad set up that runs the different clamp load calculations over a range of "k" factors and then I look at my range to determine bolt torque.
I would love to use a skidmore and qualify my torque values, unfortunately I do not have time. I am stuck with pure calculations. I was just blown away that I was being told the bolts would break, when I can clearly see they are below proof... I'm sure I will be ok, this is a low dynamic loading vehicle and I'm using really thick steel.
Again, thanks to everyone for the great feedback!
Cabbages, knickers, It hasn't got A BEAK!
RE: Torque Values (Rule of thumb?)
google "squirter washer"
RE: Torque Values (Rule of thumb?)
This program has already launched and the customer will be bolting the key joints down any day now.
I have a bit of time before production starts, so I might order some bolts and go out to our skidmore.... might... lol
Cabbages, knickers, It hasn't got A BEAK!
RE: Torque Values (Rule of thumb?)
In theory, you get the nut tight without preloading the bolt. Then you turn the nut a predetermined number of degrees, hence pulling it to a predetermined strain and thus, tension.
The strength of this method is that there is no variation due to friction, as with the torque method.
The well-known drawback is that it's difficult to control for the starting point; one technician's "finger tight" can be way different from another's.
I hope this late reply doesn't come as overkill!
RE: Torque Values (Rule of thumb?)
RE: Torque Values (Rule of thumb?)
Another reason higher torques are good is it prevents loosening. Locking features are great, but start with a properly torqued fastener and it's much less likely to vibrate loose from the start.