Use in connection with works of a temporary nature.
Note: The above masses must not exceed those marked on the blocks as being the safe mass that may be
lifted. Most blocks are limited by the size of hooks and other components and not the number of falls of rope. A factor for friction has been added.
• the safe masses shown in this table are for rope of 6 x 24 construction.
• the working load limit together with any conditions of loading deemed necessary for safe use is to be stamped or otherwise marked on each block.
• sheave diameters measured at the bottom of the groove may be as follows (temporary use only): • For power operated blocks: 15 x rope diameter
• For hand operated blocks: 10 x rope diameter
• the beckets of blocks should be steel, preferably of drop forged or wrought construction. If welded they should be to an engineered design and strongly made.
• the locking pins of hook nuts, where used, should be closely adjacent to the nut top surface. • hook shank collars should not be welded without an engineered design.
• snatch blocks should incorporate a locking pin of positive type not requiring the use of any tool for its effective positioning. A drop nose pin used as a hinge pin is recommended and the locking device must be strongly made and suitable for the intended use of the block.
• formula for calculating the load in lead rope: LL = BL + (BL x No Sheaves x % Friction) better formula is below as usually know winch capacity.
• formula for calculating a particular load in lead rope: BL = LL + (1 + sum of (sheave x % friction)) and can have different friction on each sheave if wanted.
Purchases
A wire rope reeved through sheaves to obtain a mechanical advantage is known in rigging as a ‘purchase’. Purchase and lead blocks should have the close fitting cheeks pattern, or be the dished type where the sheave is recessed into cheeks.
Self lubricating sheaves are recommended, but if reservoirs are used they should be filled periodically, and leathers and set screw washers checked for tightness.
Snatch blocks
Snatch blocks can be dangerous and should always be carefully watched. The gate must be properly closed and the split pin inserted and split open.
As the winch takes the weight, lead blocks stand up and lay into the strain. As snatch blocks stand up the split pin must be facing down and must be spread. There have been many fatal accidents because the split pin has been inserted face up and then dropped out, the gate opening allowing the hoist rope to drop out of the sheave.
The eyebolt and shackle type of block is preferable to the hook type. If the hook type is used it is important that the hook is placed into the sling with the hook facing down. If the hook faces up, it can drop out of the eye of the sling as the winch takes the strain. The hook must be properly moused to the sling.
Sheave blocks
Sheave blocks should be pulled apart, inspected and greased before each new set-up with particular attention given to the pin. If sheaves are not properly greased, friction increases dramatically through the system as the load is raised. This can overload the hoist rope at the winch.
Ensure that all cotters, nuts and bolts are tight. Lead blocks should be supported at the becket to prevent the block from twisting. Twisting would cause the rope to jam or ride on the rim of the sheave, and slip between the sheave and the cheek plate, jamming and destroying the rope.
The anchorage at the standing part of a purchase must be made at the becket at the bottom of the top block. If the becket is defective the eye of the standing part should be shackled to the head sling of the top block. Do not secure the end to the upper eye or shackle of the top block because the rope may cut where it passes over the cheek plate.
The screw pins of ‘D’ or bow shackles should be moused where used on standing rigging, and running rigging where the pin can become unscrewed, causing a serious accident.
When lifting loads by bridle or cock billing, make sure that lifting slings are ‘stopped’ and packing and lagging is lashed on. Head slings must not render or slip during fleeting operations.
The lead from the head block of any purchase must not foul its own block or any part of a structure. Head slings must be prevented from slipping by a ‘stopper’ lashing. Prevention from slipping must be against the pull from the load in the lead or from any fleeting action.
Where any fleeting action takes place the load must be kept as low as possible to the ground or any supporting structure.
During fleeting do not stand in the line of pull from either set of blocks. Many people have been seriously injured because they were in the way of a surging load.
33
How to work out the load in a single part of a purchase
The greatest load on any rope in a purchase is the load in the lead rope to the winch. This is due to the friction between the rope in the groove of the sheave and the sheave pin. Friction is estimated at between 3 per cent and 5 per cent per sheave (ie up to one twentieth of the rope load that would occur if there was no friction).
The effects of friction, acceleration or deceleration are not usually included when dealing with work of a temporary nature unless a number of falls are used or the rope velocity is high, ie. 0.6m/sec.
When a load is at rest, suspended from the lower block, the becket load (the load in each part of the rope purchase) is found as follows:
Becket load = Total load on lower block ÷ No. of parts of rope supporting load
Note: The total load on the lower block includes the load to be lifted plus packings, slings, shackles,
blocks etc.
For example – (including frictional effects)
Total load on the lower block = 10t including gear Number of parts of rope = 5 supporting the moving block
Becket load (BL) = 10 + 5 = 2t
However as lifting commences friction causes the load in the rope falls to increase by up to 5 per cent for each sheave the rope passes over, including lead sheaves (if any).
The load in the lead to the winch (given 5 per cent friction) may be calculated as follows:
Load in lead to winch (LL) = BL + (BL x number of sheaves x 0.05) = BL + (BL x number of sheaves x 5 ÷ 100)
or for a given load in the lead, the maximum load that can be lifted is calculated as: Load = LL x no. of parts supporting load
1t (number of sheaves x friction) Example 1:
No. of sheaves in purchase = 5 (3 top block + 2 bottom block) Number of parts of rope = 5 supporting lower block
Number of lead blocks = 2 (7 sheaves in total) Total load on lower block = 10t
Becket load = 10 ÷ 5=2t Load in lead rope to winch = 2 + (2 x 7 ÷ 20)
= 2.7t
Example 2:
Calculate maximum load for the above arrangement using a winch with a 2.7t line pull Load = 2.7 x 5
1 + (7 x 0.05) = 13.5 1.35 = 10t
The above calculations do not allow for sudden impact, acceleration and deceleration which can cause very high loads in the rope. These should all be avoided.
Where the angle in a lead rope is less than 90 degrees, the strain on the lead block is double the strain on the lead rope.
If the lead block is shackled to, or hooked into a sling which is reeved, the sling has to have a capacity which is four times the load in the lead rope.
Reverse bends
Avoid reverse bends because they cause much greater fatigue than if all bends were made in the one direction.
A rope running in one direction over one sheave and then in a reverse direction (ie ‘S’ fashion) over another sheave will suffer early fatigue and deterioration. As the rope passes over a sheave it is bent, and as it leaves the sheave it is straightened, two distinct actions causing fatigue. This is made worse if the rope after being bent in one direction is then straightened, and again bent in an entirely opposite direction over another sheave after which it is again straightened.
Multiple layers on drums
If a load is to be lifted to a height where multiple layers must be layed onto a drum, there are several safety precautions that should be taken.
Independent steel wire cored ropes should be used to prevent crushing. Do not use 6/37 construction ropes because the small wires will suffer badly from crushing.
The drum must have the capacity to take the amount of rope. The bottom layers must be tightly and neatly laid onto the drum.
In the absence of any test certificate it must be assumed that the rope is made from 1570 MPa and the safe working load should be calculated accordingly.
The capacity of drums and storage reels
There is a rule of thumb formula for determining the amount of rope that can be stored on a storage reel. This formula can be used when determining whether the winch drum has sufficient capacity to take the amount of rope needed in a purchase.
Length of rope that can be stored on a reel
Capacity L in metres = (A + D) x A x C ÷ 1000 x K
L = Length
A = Depth of reel flange in mm D = Diameter of reel in mm
C = Distance between flanges in mm