When towing or pulling a load up an incline the effort increases with the slope. Here are three key points that the rigging crew should address to ensure a successful operation.
1. Know the load’s weight and the rolling equipment’s CoF (coefficient of friction). You’ll notice the formulas offered from ITI’s Master Rigger’s Reference Card provide the means to calculate the required force if faced with a pull on a Level surface, Uphill or Downhill. Synthetic wheels such as provided with a common 3-point industrial rolling system offer a CoF of about .02, while steel rollers like those produced by Hilman, Multi-ton and Heavy Haul typically have a CoF of about .05. Discovering the CoF while on a level plane is especially helpful to refine the calculation for an uphill pull. Using a dynamometer in the winch line during a level pull can help finalize the approximate CoF for a particular set of rolling equipment.
2. Correctly accounting for the height, run and ramp length will help determine the values needed for either the Uphill or Downhill formula. These significantly influence the winch pull required and the sizing of the equipment; winch, snatch blocks, slings and shackles.
3. We have all heard about “staying out of the bight”, which infers being clear of inside block angles and areas that can be in the line of fire if one experiences a sling or equipment failure. Some winches have controls immediately adjacent to the winch line, so the precaution is for non-essential personnel who aren’t critical to the load pull while the equipment is under tension. In many cases, essential personnel must be next to loaded winch lines during the operation. It is a matter of minimizing the risk to the lowest level, verifying the calculations, confirming the capacity of the rigging equipment, developing and following the proper procedure and adhering to all reasonable safety restrictions where possible. Having the right equipment, well maintained and inspected is a significant key to success.
Now, let’s have some fun. Try your hand at calculating the required force (tension in lbs.) by tackling the workshop below. It is based on the adventures of Stinker and Tinker, two brothers who always seem to be getting into a rigging challenge (from Mike’s Rigging Mysteries). (Truck winches are not officially covered by ASME or OSHA.) Answers can be found below.
Determine the amount of winch pull required to pull this truck out of a small canyon. Use the Uphill formula:
[CF×W×(R/L)]+[(H/L)×W] = F
Given: The truck weighs 5,200 lbs.
Given: The run is 7' and the height is 8'.
Given: The ramp length (L) is 10.6'.
Given: The coefficient of friction is .05.
- The required winch pull is _____ lbs.
- The winch and its mounting is rated at 5,000 lbs. Is it
overloaded? Yes _____ No _____
Hint: Refer to the MRRC-P6. (While stump-jumping, Stinker ended
up sliding down into a small canyon and his truck doesn’t have the
horsepower to get out the same way he came in.)
Happy trails to all my crane and rigging friends,
1. Winch pull = [.05 x 5,200 x (7/10.6)] + [(8/10.6) x 5,200] = 172 + 3,925 = 4,097 lbs.