• Post author:
  • Post comments:24 Comments
  • Post last modified:September 25, 2021
  • Reading time:6 mins read

In many cases, the calculation of bolt torque is a very important issue, which can help us choose more suitable hydraulic tools for work. Therefore, I will show you how to calculate the bolt torque.

Now, let’s start.

Calculation formula of ordinary bolt torque:

T = KFd(N,mm)

K, tightening torque coefficient, a reference value can be roughly selected according to the surface state. For general machined surfaces, if there is lubrication, K can be 0.13-0.15; if there is no lubrication, K can be 0.18~0.21;
D, refers to the nominal diameter of the thread
F, is the pre-tightening force, carbon steel bolt F=(0.6~0.7)σA; alloy steel bolt F=(0.5~0.6)σA; where σ is the yield point of the bolt material; A=π/16(d2+d3 )2, d2 is the middle longitude of the thread, d3=d1-H/6; d1 is the minor diameter, H is the original triangular height of the thread; the H value can be calculated according to different threads, which is about 0.87 times the pitch P.
T, bolt torque.

Put the values ​​of different parameters into the formula to get the theoretical torque value of the bolt.
You can check our bolt maximum torque table to help you save time and reduce the trouble of calculation.

Calculation formula of high-strength bolts:

T = KPd

K is the torque coefficient, you can refer to the manufacturer’s test report and on-site retest report,

P is the standard value of the construction pretension value.

D is the bolt diameter.

For example

Torque calculation of high-strength bolts

High-strength bolts exert pre-tension and transmit external force by friction. Ordinary bolt connections rely on the shear resistance of the bolt rod and the pressure of the inner wall of the hole to transmit the shear force. The pre-tension force generated when the nut is tightened is very small, and its effect can be ignored. In addition to the high material strength of the high-strength bolt, it also applies to the bolt. A large pretension force produces an extrusion force between the connecting members, thereby causing a large friction force perpendicular to the screw direction. Moreover, the pretension force, the anti-slip coefficient and the type of steel directly affect the bearing capacity of the high-strength bolt.

According to the characteristics of the force, it is divided into pressure type and friction type, and the calculation methods of the two are different. The minimum specification of high-strength bolts is M12, and M16~M30 are commonly used. The performance of oversized bolts is unstable and should be used with caution in the design.

High-strength bolts refer to bolts with relatively strong grade hardness performance, also called high-strength bolts. Generally refers to 8.8 or above, such as 10.9 grade bolts and 12.9 grade bolts, which have strong hardness and torsion resistance.

In the labeling method, the number before the decimal point of the performance level represents 1% of the nominal tensile strength σb of the material. The number after the decimal point represents 10 times the ratio of the yield strength σs of the material to the nominal tensile strength. The nominal tensile strength of 8.8 performance grade of M20 bolts is σb=800MPa, and the minimum tensile strength σb=830MPa. Nominal yield strength σs=640, minimum yield strength σs=660

There is another explanation. The number before the decimal point indicates the tensile strength after heat treatment. The number after the decimal point represents the yield ratio, that is, the ratio of the measured value of the yield strength to the measured value of the ultimate tensile strength. Grade 8.8 means that the tensile strength of the bolt rod is not less than 800 MPa, and the yield ratio is 0.8. Grade 10.9 means that the tensile strength of the bolt rod is not less than 1000 MPa, and the yield ratio is 0.9.

Calculation of the length of high-strength bolts

High-strength bolt connection must strictly control the length of the bolt. The length of the torsion shear type high-strength bolt is the length from the lower supporting surface of the screw head to the cut of the screw tail; for the high-strength hexagon head bolt, the thickness of a washer should be added.

The general formula for calculating the length of high-strength bolts is as follows:


Where △L=M+NS+3P

In the formula, L—the length of the high-strength bolt;

L”—total thickness of connecting board;

△L—additional length, that is, the lengthened value of tightening length;

M—Nominal thickness of high-strength nut;

N—Number of washers, 1 for torsion shear type high-strength bolts, 2 for high-strength hexagon head bolts

S—Nominal thickness of high-strength washer

P-the pitch of the thread.

The tightening length extension value of high-strength bolts = bolt length-plate thickness. Generally, the length of the connecting plate thickness plus L is used, and the value is an integral multiple of 5mm.

Before high-strength bolt construction, the torque coefficient of the high-strength bolt connection pair should be re-examined according to the factory batch. 8 sets of each batch should be re-examined. The average value of the torque coefficient of the 8 sets should be within the range of 0.110~0.150, and the standard deviation should be less than Or equal to 0.010. After the test, the high-strength bolts should be installed in a short time.

The construction torque of high-strength bolts is calculated and determined by the following formula:


Tc—construction torque (N•m);

k—The average value of the torque coefficient of the high-strength bolt connection pair;

Pc—Pre-tension for construction of high-strength bolts (kN), see Table 1;

d—High-strength bolt screw diameter (mm);

The torque wrench used in the construction of high-strength bolts must be calibrated before use, and the torque error shall not be greater than ±5%, and it shall be used only after it is qualified. For the torque wrench used for calibration, the torque error shall not exceed ±3%.

What is the maximum tensile force of a grade 8.8 bolt?

Calculation method

The maximum stress value before the steel breaks under tension is called the strength limit or tensile strength

F = σs * A

F is the tensile force (allowable load)
σs is the tensile strength of the material
A is the effective area, and the effective area is the cross-sectional area at the smallest diameter on the effective length of the bolt.

The effective diameter of M20 is Φ17, the effective cross-sectional area is 227 square millimeters, and the minimum tensile strength of M20 of class 8.8 is σb=830MPa.

F = 830*227=188.41KN
Therefore, the minimum tensile strength of M20 bolt 8.8 performance grade is 188.41KN.

Pre-tightening torque Mt = K*P0*d*0.001N.m

K: Tightening force coefficient
D: Nominal diameter of thread
P0: preload
As: Π*ds*ds/4 (ds: the calculated diameter of the dangerous section of the threaded part)
ds: (d2+d3)/2
d3: d1-H/6 (H: nominal working height of thread)
σs: yield limit of bolt material N/mm2
K: refer to the table


TorcStark® is a professional manufacturer of hydraulic equipment. Mainly produce hydraulic torque wrenches, hydraulic bolt tensioners, hydraulic torque /tensioner pumps and other products.

This Post Has 24 Comments

  1. bugle

    Hey tһere! This is my firѕt visit to your blog!
    We are a team of volunteers and starting a new initiative in a community in the same niche.
    Yoᥙr blog provided uѕ beneficial information to work on. You
    have done a wonderful job!

  2. bluebirds

    Yⲟu’re so cooⅼ! I don’t suppose I’ve truly read a single thing like
    this before. Ꮪo wonderfսl t᧐ discover sоmeone with unique thoughts on this subject matter.

    Seriօᥙsly.. many thanks for starting this uρ. This web site is one tһing tһаt is required on the internet,
    sօmeone with a bit of originality!

  3. sheds

    Nice post. I used to Ƅe checking continuously this blog and I’m inspired!
    Very helpful information particulɑrly the final part 🙂
    I care for such info a lot. I was seeking this particular
    information for a long time. Thanks and good luck.

  4. Callie

    Hi there! This post couldn’t be written any better!
    Looking through this post reminds me of my previous
    roommate! He constantly kept preaching about this. I most certainly will send this information to him.
    Fairly certain he’ll have a good read. I appreciate you for

  5. dulls

    Howdy would you mind ⅼetting me know which
    hosting company you’re utilizing? I’ve loaded your blog in 3 different browsers and
    I must say this bloɡ loаds a lot quicker then most.
    Can you suggeѕt a good web hosting provider at ɑ
    reasonabⅼe рrice? Thаnks, I appreciate it!

  6. Zac

    Wow that was odd. I just wrote an incredibly long comment but after I clicked submit my comment didn’t appear.

    Grrrr… well I’m not writing all that over again. Anyways, just wanted to say excellent blog!

  7. Marissa

    Good day! I could have sworn I’ve visited this
    blog before but after looking at many of the posts I realized it’s new to
    me. Nonetheless, I’m certainly pleased I came across it and I’ll
    be book-marking it and checking back frequently!

  8. Wendi

    I was wondering if you ever considered changing the structure of your site?
    Its very well written; I love what youve
    got to say. But maybe you could a little more in the way of content so people could connect with
    it better. Youve got an awful lot of text for only having one or two pictures.
    Maybe you could space it out better?

  9. lowe

    Wow thаt was odd. I just wrote an very long comment but after I clicked submit my comment didn’t show
    up. Grrrr… well I’m not writіng all that over again. Anyways,
    just wanted to say great blog!

  10. Eddy

    Good article. I absolutely appreciate this website. Keep it

  11. Gladis

    Hi there, the whole thing is going sound here and ofcourse every one
    is sharing information, that’s actually good, keep up writing.

  12. Georgiana

    I do agree with all of the ideas you’ve introduced in your post.
    They are really convincing and can certainly work. Nonetheless, the
    posts are too quick for novices. May you please prolong them
    a little from next time? Thank you for the post.

  13. Dawna

    I know this if off topic but I’m looking into starting my own weblog and was curious what
    all is needed to get setup? I’m assuming having a blog like yours would cost a pretty
    penny? I’m not very internet smart so I’m not 100% certain. Any recommendations or advice would be greatly appreciated.

  14. Lindsey

    It’s very trouble-free to find out any matter on net
    as compared to textbooks, as I found this post at this site.

  15. Sammy

    This piece of writing is actually a fastidious one it assists new internet people,
    who are wishing in favor of blogging.

  16. Veronica

    I’m amazed, I have to admit. Rarely do I come across a blog that’s both educative and engaging, and let me
    tell you, you’ve hit the nail on the head.
    The issue is something too few folks are speaking intelligently
    about. Now i’m very happy that I stumbled across this during my search for something regarding this.

  17. Maier

    Wow that was strange. I just wrote an very long comment but after I clicked
    submit my comment didn’t appear. Grrrr… well I’m not writing all that over again. Anyway,
    just wanted to say superb blog!

  18. Zniak

    With havin so much written content do you ever run into any
    problems of plagorism or copyright infringement? My website has a lot of unique content
    I’ve either written myself or outsourced but it seems a lot of it is popping it up all over the internet without my authorization. Do you know
    any ways to help stop content from being ripped off? I’d truly appreciate it.

  19. Rigal

    Very shortly this web site will be famous among all blog visitors,
    due to it’s pleasant content

  20. Kerrie

    Hello! Do you use Twitter? I’d like to follow you if that would be okay.
    I’m absolutely enjoying your blog and look forward to new updates.

  21. Tracy

    Heya i’m for the primary time here. I came across this board and I to find It
    truly helpful & it helped me out a lot. I’m hoping to provide one thing back
    and help others like you helped me.

  22. vivian

    F᧐r the reаson that the admin of thіs website is working, no uncertainty very rapidly it will be renowned, due
    to its quality contents.

  23. delia

    I’m amazed, I must say. Rarely do I come across a blog that’s equally educative
    and engaging, and let me tell you, you’ve hit the nail on the head.
    The issue is something not enough people are speaking intelligently about.
    I am very happy I stumbled across this during my search for something regarding this.

  24. norber

    Good article! We will be linking to this particularly great
    content on our site. Keep up the good writing.

Leave a Reply