Chapter 15 - Fasteners: Determine the tensile load capacity of a 5/16 - 18 UNC thread and a 5/16 - 24 UNF thread made of the same material. Which is stronger?

Make the same comparison for M8 x 1.25 and M8 x 1 ISO threads. Hint: see Tables 15-1, 15-2, and 15-3

The 5/16 - 24 UNF is stronger because it has more tensile load capacity.

Tensile load capacity for M8 - 1.25 = 5670 lb

Tensile load capacity for M8 - 1 = 6067 lb

Explanation:

For 5/16 - 18 UNC thread:

D = 0.3125

n = 18

Therefore the tensile load capacity is = 100000 X (0.7854 X (0.3125 - 0.9743 / 18) ^2

= 5243 lb.

Similarly for 5/16 - 24 UNF, only the n value changes to 24

we get the tensile load capacity = 5806.6 lb

Hence the 5/16 - 24 UNF is stronger because it has more tensile load capacity.

For metric Bolts:

We have to consider all values in SI units

Strength = 689 MPa

We get for M8 - 1.25:

Tensile load capacity as = 689 X 36.6 = 25223 N = 5670 lb

For M8 - 1:

Tensile load capacity as = 689 X 39.167 = 26986 N = 6067lb

The material strength is given as 100,000 PSI

To find the tensile load capacity of a threaded rod, we multiply the Strength of the material with the Stress area for the bolt.

A_{t} = 0.7854 (D - / {0.9743}{n}) ^{2}

Where D is the Basic Major Diameter

n is the Number of threads per inch

At is the Tensile strength area of the bolt.

For 5/16 - 18 UNC thread

D = 0.3125

n = 18

Therefore the tensile load capacity is = 100000 X (0.7854 X (0.3125 - 0.9743 / 18) ^2

= 5243 lb.

Similarly for 5/16 - 24 UNF, only the n value changes to 24

we get the tensile load capacity = 5806.6 lb

Hence the fine thread bolt is stronger because it has more tensile load capacity.

For metric Bolts

We have to consider all values in SI units

Strength = 689 MPa

We get for M8 - 1.25

Tensile load capacity as = 689 X 36.6 = 25223 N = 5670 lb

For M8 - 1

Tensile load capacity as = 689 X 39.167 = 26986 N = 6067N

Strength of 5/16 - 14 Acme thread

We use the imperial fomula as mentioned above,

The tensile capacity is 4634 lb.