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Snap Hooks & Quick Links – How many Kn does it take to break ?

Disclaimer: This report describes the results of mechanical tests conducted on a limited number of unspecified chain and/or cable links. The University of Johannesburg accepts no responsibility for how the data is used and/or interpreted.

Please note: The original document was provided by the University of Johannesburg Department of Mechanical Engineering and has been edited by Climb ZA. All information is kindly provided by and copyright to the University of Johannesburg – Department of Mechanical Engineering. Date: 20 January 2005

Snap Hooks & Quick Links – How many Kn does it take to break ?

Test the Stress
The load carrying capacity of two different types of snap hook and quick link combinations were tested on an Instron 5500 Universally testing apparatus. Load and displacement were recorded and presented on graphs. The modes of failure were also recorded. From the tests conducted it can be concluded that the shear pin, as well as the machined grooves on the closing latch are critical parts in a snap hook and greatly influences its load carry capacity.

Introduction
Snap hooks and quick links (Appendix A) are used in combination with chains or cables to lift equipment. Two combinations of a snap hook and quick link were tested to destruction (10×100 mm and 8 mm , as well as 9×90 mm and 6 mm). Snap hooks are fitted with a latch that is by default in the closed position. Machined edges at the front end of the latch fits into groves. This feature has the twofold purpose of providing additional strength (providing additional resistance to the snap hook from bending open when loaded), and facilitates chain or cable loading. Once closed the latch prevents the cable or chain from accidentally falling from the hook when no load is applied. The aim of the investigation is to determine the load carry capacity of two different size snap hooks (9×90 mm and 10×100 mm) in combination with a quick link (6 mm and 8 mm). The mode of failure will also be investigated.

Experimental results
Two different snap hook and quick link combinations were tested (Appendix A):

  1. A 10×100 mm snap hook (similar to SHEZ010 in Appendix A) with a 10 mm diameter quick link (similar to QLZ008 Appendix A), and
  2. An 8×80 mm snap hook (similar to SHEZ008 in appendix A) with an 8 mm diameter quick link (similar to QLZ006 in Appendix A).

The load and displacement data of the different combinations are presented in Figure 2 and 3. Table 1 presents the average failure loads. Four of the larger and one of the smaller combinations were tested.

Modes of failure
The shear pin in the larger snap hook failed in all four tests conducted. The average calculated shear stress was (2.4 mm diameter pin) 1196 MPa (Figure 4). The single smaller combination tested failed in the grooves machined into the snap hook latch. The average failure stress at the grooves was calculated to be 640 MPa (Figure 5). The quick links showed no evidence of failure in any of the tests conducted. After initial failure the load carrying capacity of the specimens fall dramatically and the snap hook starts to open. The displacement then increases significantly with only a small increase in load carrying capacity. This mode of failure is typical for both types tested with only the magnitude of load varying.

Conclusion
From the tests conducted it can be concluded that the shear pin and the grooves on the closing end of the latch are critical parts. The larger snap hook, quick link combination failed at an averaged load of 10.8 kN while the smaller combination failed at 9.3 kN. The factor of safety for the larger snap hook was 2.35 based on a maximum prescribed load of 4.6 kN. After initial failure the load bearing capacity drops to approximately 50 % of the initial load. During this stage the unsupported mouth progressively opens up.

Specimen
combination
Measured Load (kN)
Test 1 Test 2 Test 3 Test 4 Average
10×100,
10
Initial maximum load 10.9 11.4 11.1 10.0 10.8
Secondary load attained after initial failure * 5.1 5.2 4.7 5.0
8×80,
8
Initial maximum load 9.3 9.3
Secondary load attained after initial failure 3.4 3.4
Table 1. Experimentally measured failure loads

Table 1. Experimentally measured failure loads

Figure 2.
Graph of load versus displacement for the 10×100 mm snap hook, 10 mm quick link combination

SNAP HOOK WITH EYELET,
ZINC PLATED
ART. NO. SIZE/MM WLL/LBS
SHEZ004 4 x 40 200
SHEZ005 5 x 50 220
SHEZ006 6 x 60 260
SHEZ007 7 x 70 400
SHEZ008 8 x 80 500
SHEZ009 9 x 90 550
SHEZ010 10 x 100 770
SHEZ011 11 x 120 990
SHEZ012 12 x 140 1200
SHEZ013 13 x 160 1400
SHEZ014 14 x 180 1600
QUICK LINK, ZINC PLATED
ART. NO. SIZE/MM WLL/LBS
QLZ003 3 220
QLZ035 3.5 375
QLZ004 4 500
QLZ005 5 660
QLZ006 6 880
QLZ007 7 1300
QLZ008 8 1760
QLZ009 9 2200
QLZ010 10 2640
QLZ011 11
QLZ012 12 3300
QLZ014 14 6100
QLZ016 16

Appendix A: Abstract from products list of H-lift Industries CO. LTD.

Figure 1. Specimen in grips

Figure 1. Specimen in grips

Experimental Set-up
An Instron 5500 Universal testing apparatus was used to apply the load. This machine has the capability of providing a tensile and compressive force in excess of 100 kN. A 100 kN load cell was used to measure the force exerted. Load displacement transducers in the Instron 5500 measures displacement. The screw driven frame arm of the Instron tester was set to move at a rate of 5 mm/min. The resistance to this movement is the applied load and was recorded by the load cell. The load cell data and corresponding displacement were captured with a dedicated data recorder device, which is part of the basic Instron 5500 set-up. Grips were used to fix the tested specimens between the frame arm and the load cell. To ensure that the load is applied in a similar manner as it would be in a practical application two pieces of flat bar with holes at the ends were used to transmit the load between the grips and the specimens (Figure 1). The holes had a fillet to make certain that no undue stress concentrations were induced in either the flat bar or the specimens. The grips are capable of rotating with respect to one another. This ensured that the snap hook and quick link could be at an angle of 90 ° with respect to each other.

Figure 4. Mode of failure on 100 mm long, 10 mm diameter snap hook

Figure 4. Mode of failure on 100 mm long, 10 mm diameter snap hook

Figure 5. Mode of failure on 95 mm long, 8 mm diameter snap hook

Figure 5. Mode of failure on 95 mm long, 8 mm diameter snap hook

One Response to Snap Hooks & Quick Links – How many Kn does it take to break ?

  1. Cuan Oct 1, 2011 at 11:09 am #

    Hi guys, nice article…Just that your Kn in the title should read kN so some of us don’t read it as Kelvin nano :P Cheers

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