JOINING OF SNATCH STRAPS

An illustration of why Snatch Straps and/or Winch Extension Straps MUST NEVER be joined with shackles.

Determine Characteristics of 8000kg Breaking Strain 9m Snatch Strap.

A 550kg Test Weight was hung from the Snatch Strap producing 260mm Elongation.

From Hookes Law

Stretching Force, F = kx
where k = spring constant
x = distance stretched (meters)
(550kg) (9.81) = k (0.260m)

therefore
k = 20.75kN/m
or k = 2.075 Tonne/m

Determine Energy Produced by a Recovering 4WD.

For a 2.5 Tonne 4WD travelling at 6km/h,
The Kinetic Energy available, KE4WD = 1/2 m v2
where

m = mass of 4WD (kg)
v = velocity of 4WD (m/s)

KE4WD = 1/2 (2500kg)[(6km/h) (1000/3600)]2
KE4WD = 3.47kJ

Checking Stretch of Snatch Strap.

If all the Kinetic Energy of the Recovering 4WD is transferred to Elastic Energy in the Snatch Strap then the stretch of the strap can be determined. (Assuming no losses due to friction)

From Energy Conservation,

Kinetic Energy of Recovering 4WD = Elastic Energy in Snatch Strap
KE4WD=PEStrap
3472 J = 1/2 k x2
therefore x = 0.579m
or approximately 6% extension.
And from Hookes Law the Force Stored in the Snatch Straps is,
F = kx
= (2.075 Tonne/m) (0.579m)
= 1.2 Tonne

Determine Speed of Shackle if Snatch Strap Breaks.

If all the Elastic Energy of the Snatch Strap is transferred to Kinetic Energy in the Shackle then the peak Speed of the Shackle as the Snatch Strap returns to it's 9.0m length can be determined. (Assuming no losses due to friction)

From Energy Conservation,

Elastic Energy in Snatch Strap = Kinetic Energy of Shackle
PEStrap = KEShackle
1/2 k x2 = 1/2 m v2
where k = spring constant
x = distance stretched (meters)
m = mass of shackle (kg)
v = velocity of shackle (m/s)
1/2 (20750N/m) (0.579m)2 = 1/2 (0.68kg) v2
v = 101m/s

or Peak Speed of Shackle   v = 364 km/h
 This shackle speed gives no time for any reaction to get out of the way and would cause tremendous damage to whatever it contacts.

Snatch Straps and/or Winch Extension Strap should always be joined using the loops on the ends of the Straps as illustrated below.

References: Introduction to Physics for Scientists and Engineers. By Frederick J. Bueche