Soft attachment blocks provide a number of unique advantages over mechanically fastened pulleys. The foremost is versatility. Lashing blocks can be secured in diverse locations, without hardware or metal fasteners—just a rope loop.
However, this does raise a few questions about the integrity of the 3-part system (the block, the loop, and the attachment point) that will need attention. These components may have wildly different load tolerances.
- What is the maximum load rating on the lashing block?
- What about the particular natural or synthetic fiber rope of the loop?
- Is the pipe, beam, hook, or other attachment points for the loop capable of safely bearing the same maximum load as the hardware and rope?
- Even if you know the maximums, have you calculated the safe working load vs. breaking strength of each part?
The answers to these questions may require adjustments in your pulley system.
Safe Working Load vs. Breaking Strength
Industrial grade pulley systems are load-tested, meaning they have undergone a series of performance tests to determine how well they function under a full range of conditions and where their limits lie. The load rating of a piece of equipment represents the maximum load that a particular piece will carry before the item fails or breaks. This is also called the breaking strength.
However, operating at or near this limit can often damage equipment over time, reducing strength and safety. The safe working load is a lower limit at which the equipment will function without stressing or degrading the material (requiring eventual replacement). If you exceed the safe working load, this may mean that the item will bend or deform but not fail.
Ronstan lists both of these figures on all of our parts to help you determine which is right for you. A piece of hardware that is not load rated means it comes with no guarantee it can carry up to a specific load. Non-rated components, while typically cheaper, aren’t the best option for applications in which customer satisfaction and safety are paramount.
Lashing Blocks and Loops
The tenacity of a rope is a measure of the total force the fiber can withstand before it snaps. The difference between the breaking strength of a rope and the recommended working load is often significant.
In general, ropes shouldn’t typically operate under loads more than 20% of their breaking strength. Continuous stress near the limits of a rope’s strength can weaken it over time. Also, the closer the rope is to its maximum load, the less margin of error there is in the case of a mishap that puts a sudden shock load on the rope.
Synthetic ropes have grown extremely strong in this regard, in many cases surpassing the strength of steel weight-for-weight. Here are a few comparisons of synthetic fibers:
- Aramids: 5 times steel strength
- Liquid Crystal Polymer: 5 times steel strength
- Polybenzoxazole: 10 times steel strength
- High Modulus Polyethylene (HMPE): 15 times steel strength
However, not all of these fibers tolerate the pinches and turns required of lashing knots, and the tight turns of a soft attachment loop equally well. Aramid fibers, for instance, have a hook shape that creates internal abrasions and weakens the rope significantly when stressed around a tight turn (or within a knot).
HMPE, by comparison, tolerates tight turns very well. Combined with its high tenacity, it’s very suitable for use as a lashing block loop. The last thing you want in a lashing block loop is for the ratio of safe working load vs. breaking strength to be compromised by degraded fibers.
If you’re interested in crafting your own lashes and loops, Ronstan offers Ocean 3000 and Ocean 5000 Dyneema® ropes made from optimal-performance HMPE fibers. We also carry reliable, load-rated lashing blocks that feature the best breaking strength in their class.