Torque to Bolts

 

Bolts and screws are so ubiquitous that it’s impossible to fully articulate their importance or applications. These fasteners literally hold the world around us together. From the rigors of industry, to the rumble of cars, trains and planes, to the furniture that adorns our homes and offices, bolts play an integral role in assembling the materials that structure our lives.

            Perhaps it’s their ubiquity that makes bolts so underappreciated as a mechanical component. Too often bolt selection is made in haste. The purchaser believes he or she has their assembly challenge met after considering just a few parameters. What shank diameter and length do I need? Metric or imperial? Thread pitch?

            When a bolt-fastened joint fails, not only have the workpieces been ruined, but the purchaser is none the wiser as to why that joint failed. Worse yet, a dissatisfied customer often shifts blame to faulty hardware or a mediocre supplier.

            But a supplier with superior product knowledge and exceptional customer service can make an enormous impact on your company’s bottom line. Bayou City Bolt has more than 50 years of experience with helping customers find the right hardware for any application. Fastener orders from Bayou City Bolt are on time; bolts are always to specification; and they are affordable for any organization.

 

Why Torque Matters

            Why did that joint fail? More than likely, it was an issue of inadequate torqueing. Proper torqueing is vital to the function of the bolt and is determined by several, often conflicting factors.

            A properly tightened bolt has its material stretched slightly, but not beyond its elastic limit. The bolt material, most commonly steel, resists this natural stretching and creates a clamping force upon the assembled substrates. Similarly, the substrate materials resist compression to balance the clamping pressure; this is known as the joint preload. A correctly-tightened bolt shares the preload with workpieces.

An over-torqued bolt that is stretched beyond its elastic limit is severely weakened, diminishing its effective load capacity. An under-torqued bolt or screw will allow negligible separation between the workpieces, which seems trivial at first, but after consistent dynamic loading or other operating stresses, the gap between workpieces will grow. A gap in the joint represents no joint preload. Without the return force of the compressed substrates, the bolt is solely relied upon for joint assembly—a condition that inevitably leads to joint failure.

 

Determining Proper Torque

Even experienced tradesmen over- or under-torque bolts. Truthfully, product information rarely supplies torque values. Common bolt torque values can be looked up, but finding an accurate reference isn’t always easy.  Bolt torque can be checked with a tool such as a torque wrench, but without a value as a guideline, a torque wrench offers no advantage. To arrive at the correct torque value, several other values must be found first.

Two principles influence the correct clamping pressure for each bolt, known as clamp load. The first is bolt diameter. The second is the class of the bolt, defined by the bolt’s tensile strength, which in turn is determined by the material of construction. Thankfully, standards organizations have assembled the standard tensile strengths for common bolts into easy-to-use standards. SAE J429 governs imperial sizes, while ISO 898 governs metric-size bolts.

SAE J429



Bolt grade

Bolt Material 

Bolt diameter

Minimum tensile strength

Grade 2

Low- to medium-grade carbon steels

1/4 to 3/4in.>3/4 to 1 1/2in.

74,000 psi

60,000 psi      

Grade 5

medium carbon steels that have been quenched and tempered

1/2 to 1 in. > 1 to 1 1/2 in

120,000 psi

105,000 psi

Grade 8

medium carbon steels that have been quenched and tempered

1/4 to 1 1/2 in. 150,000 psi
Grade 18-8 Stainless steel 1/4 to 1 1/2 in. 65,000 psi

 

ISO 898



Bolt class Bolt material Bolt Diameter Minimum tensile strength
Class 8.8

medium carbon steels that have been quenched and tempered

<16 mm

16 to 72 mm

800 MPa

830 MPa
Class 10.9

alloy steels that have been quenched and tempered

5 to 100 mm 5 to 100 mm
Class 12.9

alloy steels that have been quenched and tempered

1.6 to 100 mm 1220 MPa
Class A-2 stainless steel All thru 20 mm 500 MPa

For imperial bolts, grades 5 and 8 are most common. SAE J429 conforming bolts will have radial markings machined on the head of the bolt that indicate bolt grade. A grade 2 bolt has no markings, a grade 5 bolt will have three markings, while a grade 8 bolt will have six lines. Metric bolts are more simply identified: the class is explicitly stamped on the bolt head.

            Other standards regulate specific types or applications for bolts and they should be consulted as needed. Examples include, but are not limited to those in the accompanying table.

Standard         

Bolt specifications

Bolt diameter

Minimum tensile strength

ASTM A325

Standard specification for structural bolts, steel, heat-treated

½ to 1 in.

>1 to 1½ in.

120,000 psi

105,000 psi

ASTM A490

Standard specification for structural bolts, alloy steel, heat-treated

½ to 1½ in.

150,000 psi

ASTM A193 B7

Standard specification for alloy-steel and stainless steel bolting for high temperature or high pressure service and other special purpose applications

 Up to 2½ in.

 

>2½ to 4 in.

125,000 psi

 

115,000 psi

Utilizing bolt class information, the clamp load of the bolt can be determined with the following equation.

 

 

P = St x As

 

Where:

P: clamp load

St: bolt tensile strength

As: tensile stress area

The value for tensile stress area can be determined from:

 

As = π/4 x (D - (.938194 x p))^2

Where:

D: bolt diameter

p: 1/threads per inch (TPI)

Clamp load is typically around 75% of a bolt’s proof load; that is, the highest stress the bolt can accommodate before experiencing plastic deformation. The proof load itself is usually 85% to 95% of a bolt’s yield strength, but the clamp load is significant because it is what ultimately provides the clamping pressure. Once the clamp load is determined, finding the correct torque value for a bolt is one simple calculation away.

T = K x D x P

Where:

K: coefficient of friction (as determined by bolt surface treatments)

Common coefficient of friction values:

Bolt surface

 K

Nonplated, black finish

0.3

Zinc plated

0.2

Lubricated

0.18

Cadmium plated

0.16

As an example, we can utilize the equation to find the correct torque value for a zinc-plated, heavy structural bolt, in this instance belonging to ASTM A325 with a ¾ inch diameter and 10 TPI.

As = π/4 x (3/4in. - (.938194/10))^2

As = .3383 in^2

Using this value, the clamp force can now be identified.

P = 120,000 x .3382

P= 40,684 lb.

And finally, the torque value for this bolt can be

T = .2 x 3/4 x 40,684

T = 6,087 in. - lb

In many ways Bayou City Bolt is like the bolts we sell: built with integrity, completely reliable, and a fundamental part of operational stability. On top of an unrivaled catalogue of hard-to-find fasteners with the lowest minimums, Bayou City Bolt is the premier hardware supplier in the southern United States. See for yourself what our customers already know: that the smallest components can make the biggest difference in every industry.

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