How to Calculate Eye Bolt Diameter to Lift Load?

A lifting eye bolt, as shown in the below picture, is used for lifting and transporting heavy machines. It consists of a ring of the circular cross-section at the head and is provided with threads at the lower portion for screwing inside a threaded hole on the top of the machine. But how to choose the correct size of the Eye Bolt for your application? How to calculate the Eye Bolt Diameter based on the load?

We have been discussing the stresses in threaded bolts in the previous article. Now, this helps us to calculate the required diameter of the Eye Bolt to lift the load that we need to lift. And also helps us to choose the Eye Bolt size.

Tensile Stress in Eye Bolt

Typically we have seen these eye bolts in Lifting a machine for transporting or pulling a car with another vehicle using the towing hook. In all of these applications, the threaded portion of the Eye Bolt will experience tensile stress due to the external force.

We know that the load that can be applied on the Bolt is calculated by the product of the permissible stress and Cross-sectional area

P = Permissible Tensile stress × Cross-sectional area of the Bolt

Let us say
σ_t = Permissible tensile stress for the eye bolt material
d_c = Root or core diameter of the thread
n = number of bolts

We can write it as

From the above relation, if we have the amount of load and permissible tensile stress of the material of the Eye bolt, the core diameter (d_c) of the Eye Bolt can be determined.

What are the design parameters for the Eye Bolt?

With the many experiments and testing done by engineers, Engineers have standardized the dimensional relation for the eye bolt design. All of these dimensions are depended on the Nominal diameter of the bolt, which can be identified from the Standard Design dimensions of the screw threads chart mentioned below.

Following is the schematic diagram for the eye bolt design dimensions.

From the above dimensions, we can design an Eye Bolt based on the given load need to be lifted. also, we need to have the permissible tensile stress of the eye bolt material.

Now let us take an example of how we can solve an example problem on the eye bolt diameter calculation and the other parameters of the eye bolt.

Example Problem on for finding Eye Bolt size and Eye Bolt design parameters

An eye bolt is to be used for lifting a load of 60 kN. Find the nominal diameter of the bolt, if the tensile stress is not to exceed 100 MPa. Assume coarse threads.

Answer:

Given Data
A static Lifting Load P = 60kN = 60×10³N
Permissible Tensile stress σ_t = 100 N/mm²

From the above relation

60×10³ = (π /4)×(d_c)² × 100 × 1
(d_c)² = 60000/78.55
(d_c)² = 764
d_c = 27.6

Where d_c is the core diameter of the bolt size. now we can get the nominal diameter of the bolt from the standard design screw threads table below.

For the value of 27.6mm, the next closest d_c value from the table is 28.706. The respective nominal diameter for this core diameter d is 33 and the bolt size is M33 bolt.

Now we have d = 33, From this, we can calculate the remaining parameters of the Eye Bolt. The following image shows all the calculated standard design parameters for the eye bolt.

Exercise Problems

1. An eye bolt is to be used for lifting a load of 30 kN. Find the nominal diameter of the bolt, if the tensile stress is not to exceed 100 MPa. Assume coarse threads from the below tables.

2. Four eye bolts are to be used for lifting a load of 280 kN. Find the nominal diameter of the bolts, if the tensile stress is not to exceed 100 MPa. Assume coarse threads from the below tables.

3. A machine of load 2534kN is to be lifted with help of eye bolts screwed on a rectangular frame. When we need to find out how many eye bolts are needed and what is the size of the eye bolt. The tensile stress should not exceed 90MPa. Assume coarse threads from the below tables.

Solve these and let us know the answers in the comment section below. I will help you.

Standard Dimensions of Screw Threads for Bolt and Nuts

According to IS: 4218 (Part III) 1976 (Reaffirmed 1996)

Coarse Threads Series

Designation	Pitch mm	Major or nominal diameter  for Nut and Bolt (d = D) mm	Effective or pitch diameter  for Nut and Bolt (dp) mm	Minor or core diameter (dc)  for Bolt in mm	Minor or core diameter (dc)  for Nut in mm	Depth of thread  for bolt -mm	Stress area mm2
M 0.4	0.1	0.400	0.335	0.277	0.292	0.061	0.074
M 0.6	0.15	0.600	0.503	0.416	0.438	0.092	0.166
M 0.8	0.2	0.800	0.670	0.555	0.584	0.123	0.295
M 1	0.25	1.000	0.38	0.693	0.729	0.153	0.460
M 12	0.25	1.200	1.038	0.893	0.929	0.158	0.732
M 1.4	0.3	1.400	1.205	1.032	1.075	0.184	0.983
M 1.6	0.35	1.600	1.373	1.171	1.221	0.215	1.27
M 1.8	0.35	1.800	1873	1.371	1.421	0.215	1.70
M 2	0.4	2.000	1.740	1.509	1.567	0.245	2.07
M 22	0.45	2.200	1.908	1.648	1.713	0.276	2.48
M 25	0.45	2.500	2.208	1.948	2.013	0.276	3.39
M 3	0.5	3.000	2.675	2.387	2.459	0.307	5.03
M 35	0.6	3.500	3.110	2.764	2.850	0.368	6.78
M 4	0.7	4.000	3.545	3.141	3.242	0.429	8.78
M 4.5	0.75	4.500	4.013	3580	3.688	0.460	11.3
M 5	0.8	5.000	4.480	4.019	4.134	0.491	14.2
M 6	1	6.000	5350	4.773	4.918	0.613	20.1
M7	1	7.000	6.350	5.773	5.918	0.613	28.9
M8	1.25	8.000	7.188	6.466	6.647	0.767	36.6
M 10	1.5	10.000	9.026	8.160	8.876	0.920	58.3
M 12	1.75	12.000	10.863	9.858	10.106	1.074	84.0
M 14	2	14.000	12.701	11.546	11.835	1.227	115
M 16	2	16.000	14.701	13.546	13.835	1.227	157
M 18	2.5	18.000	16.376	14.933	15.294	1.534	192
M 20	2.5	20.000	18.376	16.933	17.294	1.534	245
M 22	2.5	22.000	20.376	18.933	19.294	1.534	303
M 24	3	24.000	22.051	20.320	20.752	1.840	353
M 27	3	27.000	25.051	23.320	23.752	1.840	459
M 30	3.5	30.000	27.727	25.706	26.211	2.147	561
M 33	3.5	33.000	30.727	28.706	29.211	2.147	694
M 36	4	36.000	33.402	31.093	31.670	2.454	817
M 39	4	39.000	36.402	34.093	34.670	2.454	976
M 42	4.5	42.000	39.077	36.416	37.129	2.760	1104
M 45	4.5	45.000	42.077	39.416	40.129	2.760	1300
M 48	5	48.000	44.752	41.795	42.587	3.067	1465
M 52	5	52.000	48.752	45.795	46.587	3.067	1755
M 56	5.5	56.000	52.428	49.177	50.046	3.067	2022
M 60	5.5	60.000	56.428	53.177	54.046	3.374	2360

Fine Thread Series

Designation	Pitch mm	Major or nominal diameter  for Nut and Bolt (d = D) mm	Effective or pitch diameter  for Nut and Bolt (dp) mm	Minor or core diameter (dc)  for Bolt in mm	Minor or core diameter (dc)  for Nut in mm	Depth of thread  for bolt -mm	Stress area mm2
M 0.4	0.1	0.400	0.335	0.277	0.292	0.061	0.074
M 0.6	0.15	0.600	0.503	0.416	0.438	0.092	0.166
M 0.8	0.2	0.800	0.670	0.555	0.584	0.123	0.295
M1	0.25	1.000	0.838	0.693	0.729	0.153	0.460
M 1.2	0.25	1.200	1.038	0.893	0.929	0.158	0.732
M 1.4	0.3	1.400	1.205	1.032	1.075	0.184	0.983
M 1.6	0.35	1.600	1.373	1.171	1.221	0.215	1.27
M 1.8	0.35	1.800	1.573	1.371	1.421	0.215	1.70
M2	0.4	2.000	1.740	1.509	1.567	0.245	2.07
M 2.2	0.45	2.200	1.908	1.648	1.713	0.276	2.48
M 2.5	0.45	2.500	2.208	1.948	2.013	0.276	3.39
M3	0.5	3.000	2.675	2.387	2.459	0.307	5.03
M 3.5	0.6	3.500	3.110	2.764	2.850	0.368	6.78
M4	0.7	4.000	3.545	3.141	3.242	0.429	8.78
M 4.5	0.75	4.500	4.013	3.580	3.688	0.460	11.3
MS	0.8	5.000	4.480	4.019	4.134	0.491	14.2
M6	1	6.000	5.350	4.773	4.918	0.613	20.1

Conclusion

We have discussed how the Eye Bolt diameter can be calculated based on the load given and the permissible tensile stress. Also discussed are the Eye Bolt design parameters with example problems. Let us know what do you think about these in the comment section below.