Twist and shout

The rope twists… and you shout. You have a multi-part line and the block starts spinning when you pick up the load. Once this happens, you find the rope intertwining on itself and it is often necessary to stop the lift because the rope may be damaged as it rubs against itself. What happened? Usually it’s a combination of factors. Essentially, the problem can be traced to rope selection, operating parameters or wire rope installation procedure. We’ll review each of these possibilities so that you can understand how to resolve your particular block-spinning problem.

Rope selection

Operating parameters determine rope selection. The length of fall (boom tip to the pick up point) and number of parts of line (and the their spacing) determines what rope construction needs to be used. The most stable situation is to have a large spacing between the falls and a short length of fall. Stability decreases as you increase the length of fall and/or lessen the rope spacing. This is why tower cranes use bicycle blocks to have a large spacing between the ropes, thereby creating a stable rope environment. Different rope constructions have differing rotational characteristics under load so it is necessary to select a rope construction that can work properly in its intended application.

The first thing you need to realise is that all wire ropes rotate to some degree. Typical wire ropes on cranes can range from a turn value of 42°/rope lay on a 6x25F wire rope construction to a low of 0.2°/rope lay on a multi-strand Dyform 34LR, which is an example of a typical compacted strand rope. Their respective torque factors are 8.9% and 0.76%. (Values and factors taken at 20% of minimum breaking force.) Turn is measured with the rope being loaded and unrestrained. Examples of these rotation characteristics are shown in the graph on page 41. Torque is measured with both ends of the rope fixed.

  One of the more misleading terms used in the industry, is ‘non-rotating’ to describe a particular wire rope construction. In the USA this description has been changed to ‘rotation resistant’, which more accurately describes these ropes. Their resistance to rotation is schieved by having their external layer of strands laid in one direction, while the inner rope has its strands laid in the opposite direction.

As you can see from the graph, there are clear differences in rotation resistance between rope constructions. The 6x19 class wire rope is not considered rotation resistant but it does have some rotation resistance. This occurs when the rope tries to unlay under load. Since the rope is a Regular lay construction (where the wires in the six outer strands are opposite the rope lay), the wires in the strand tighten up and offer some resistance to rotation. If the 6x19 rope was a Lang lay construction (where the wires in the six outer strands are in the same direction as the rope lay) the wires in the strand would also open as the rope unlayed, thereby offering no resistance to rotation. This is why 6x19 class Lang lay wire rope is not used as crane hoist rope.

Rotation resistant classification begins with the 8x19 class, Regular lay wire rope with an independent wire rope core (IWRC). Though an improvement over the 6x19 class, it is far from being a neutrally balanced rope construction.

A popular rotation resistant rope is the 12 outer strand Dyform-18. Here the inner rope is larger and is better able to neutralise the turning effect of the outer strands. The improvement in rotation resistance makes this construction suitable for many of the small to mid-range hydraulic cranes on the market and it is often listed in the crane manufacturer’s specifications.

The best rotation resistant constructions have more outer strands and more than one layer of inner strands. As you increase the number of outer strands, the diameter of the strands becomes smaller. This allows the inner rope to be larger to a point that can neutralise the turn effects of the outer strands; and in the case of these aforementioned rope constructions, achieve a situation of almost zero turn. As a result, it is possible to convert these constructions from Regular lay to Lang lay, creating a rope which not only has superb rotational characteristics but which also offers trouble-free spooling.

Generally, the smooth outer wires of a compacted strand rope construction do not have the same mechanical interference as conventional wire rope constructions and would normally spool well. But now that they are available in a Lang lay construction, even better spooling is assured. The Lang lay rope will not have any mechanical interference at maximum fleet angle, which would prevent turn inducement during the hoisting cycle. A problem that could cause a rope to develop enough turn to cause block spinning even though the rope construction matches the operating parameters.

So how does one choose the proper wire rope construction? The easiest way is to work with a reputable wire rope distributor. They generally have a good relationship with one or more wire rope manufacturers and can get assistance from their engineering staff. Some of these manufacturers have proprietary computer programs that can account for all the variables, some of which are complicated.

Many wire rope catalogues specify the rotation resistant characteristics of each of their ropes. The specific torque and turn numbers, however, will be needed to make the proper calculations to determine if these wire ropes will be suitable for your application. Better wire rope catalogues contain the formulae needed for simple calculations from two to several parts of line. These assume simple reeving set-ups with uniform rope spacing. If you have non-uniform rope spacing or are using a cross or skip reeving setup, then a wire rope engineer will be needed to compute the calculations for you. This will also be beneficial as there may be design factor issues needing a greater rope breaking force and a wire rope engineer would be best equipped to make the proper evaluation.

There are many different rotation resistant wire rope designs on the market today and you are probably trying to determine which is the best of them all. Due to different manufacturing techniques and design, many of the specialty wire ropes are close to zero rotation under load, or at least claim to be. Ultimately, the wire rope manufacturer’s torque/turn tests on finished wire ropesverify if they meet their claims. If you are experiencing a rotation problem, check to see if the rope has been tested for its torque/turn characteristics.

The crane manufacturer has usually already selected a rope construction for a particular crane model and this should generally be suitable for most lifting applications. Where you may run into problems is operating the crane outside its usual parameters. An example is a pick point that is below the crane’s structure. This creates a greater length of fall than the crane manufacturer calculated for and may demand a wire rope modification or perhaps a change in rope spacing.

Travelling block selection can also affect the performance of a rotation resistant rope. If a block is operated with odd part reeving (3, 5, 7, etc.) and its rope termination point is at the side of the block, it will not hang plumb. If the block is misaligned, it will induce twist into the rope during operation by the rope climbing the flange or rolling into the sheaves of the travelling block. This can cause block rotation or even worse, structural upset of the wire rope. If you have a block that has the side termination, it is recommended to add a part so that you are operating with even parts of line. It will reduce the hoisting speed but can help avoid problems. If you are in the market for a new block, consider a termination point in the centre of the block.

Installation

Incorrect wire rope installation can account for a block turning problem. It is crucial to install the wire rope in such a way as to retain the as-manufactured condition of the rope. Installing the rope in a manner that induces turn will cause the rope to be extremely lively and unpredictable in its behaviour. Things like reel positioning, proper unwinding, tensioning, and rope to rope termination are all critical for a successful installation. It is these variables that cause nuances from rope to rope, which explains why one rope works and the other does not. Follow the wire rope manufacturer’s installation procedure closely to avoid problems.

If you are a victim of block spinning, your solution may be as simple as changing the wire rope construction. Or, you may have such a great length of fall with inadequate rope spacing that may be so onerous that no existing rotation resistant rope could possibly work. This may need increased rope spacing or use of another crane altogether. What is for sure is that this cannot be done alone. You will need help from a wire rope engineer or you will be doomed to frustration by trial and, mostly, error.