Rope

Pulley Structure And Efficiency

A pulley is a wheel which rotates on a shaft which is supported by a frame. They can either be used to change the direction of a rope or provide mechanical advantage to make it easier to move or lift an object. In rope systems, pulleys are primarily used to build mechanical advantage systems for raising operations.

The wheel of a pulley, called a sheave, is specially designed to accept a rope without damaging it. Most sheaves are made out of aluminum but stainless steel versions are also available for use with metal cable.

Sheave Diameter

The size of a pulley is determined by the inside diameter of the sheave. This is where a rope or cable contacts the pulley. For example, in a 2 inch pulley, the rope turns around a 2 inch diameter circle inside the pulley. The correct size of the pulley is determined by the rope that is being used. Traditionally, pulleys were very large. It was common for the sheave to be 8 times the size of the diameter of the rope. This is because natural fiber ropes, like manila, can be damaged by bending too tightly. Modern nylon and polyester ropes are much more flexible and do not suffer from this problem.

For modern ropes, we recommend a sheave that is at least 3 times the diameter of the rope. For example, for a half inch rope, a 1.5 inch pulley is recommended. This provides a good balance of efficiency and size. If additional efficiency is needed a slightly larger diameter, such as 4 times the rope diameter, can be used. Larger sheave sizes add additional efficiency but the pulley also becomes much larger and heavier.

Sheave Width

It is also important to pay attention to the width of the sheave. There is no problem using a smaller rope on a wide sheave but a rope larger than the sheave width is not recommended. Even if the rope can fit in the pulley, under a heavy load the rope can flatten and become wider. This will cause the rope to rub against the side plates of the pulley and greatly reduce efficiency.

Bushings and Bearings

The sheave rotates on the shaft which allows it to turn. It does this by using either a bearing or a bushing. These come in a few different forms:

Plastic Bushing: Bushings made out of plastic are the cheapest type. These are good for very light loads but under load they can deform causing the pulley to jam and stop turning. These types of bushings are not recommended.

Oilite Bushing: Oilite is a porous bronze or iron which is impregnated with an oil lubricant. It was originally designed for use in water pumps. These are better than plastic bearings since they will not deform under heavy loads but still have problems. Since they are not sealed, dirt can get inside the bearing and cause increased friction. Pulleys are not designed to be taken apart so cleaning out the dirt can be very difficult. The main advantage of oilite bushings is they are less expensive than sealed ball bearings. For lighter loads and if properly maintained they are a good option.

Sealed Ball Bearing: Bearings are more complicated than bushings and consist of many small balls which minimize friction. One of the main advantage of a sealed ball bearing is there is no maintenance needed. Sealed ball bearings are also much more efficient than oilite bushings.

Efficiency

The efficiency of the pulley is a major consideration when buying a pulley. Pulleys using oilite bushings are approximately 70-80% efficient. The same size pulley using a ball bearing is approximately 90-95% efficient. As the load on a pulley increases, the pulley's efficiency drops. A badly designed pulley, or one being used in a way it is not designed, can make it more difficult to lift a load than no pulley being used at all.

There are many considerations in choosing a pulley and many of them can not be seen by looking at a picture online. Often they only become noticeable during actual use.

There are some pieces of equipment where it is alright to save money but we recommend purchasing the highest quality pulley within budget. Many pieces of equipment can be improvised but a proper pulley is not one of them.

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Equipment Costs and Training Expenses

There is a balance between the cost of equipment and the cost spent on training. There is additional cost spent on training to remain proficient with the use of equipment. All of these costs must be considered when making a purchase. Something inexpensive might look like a good deal but when the total cost of ownership is considered it might end up costing much more than investing in the correct gear in the beginning.

CMC MPD Multi-Purpose Device

Development of modern rope equipment began shortly after the end of World War II and the rate of new equipment and ideas continue to grow every year. New ways are thought of to accomplish tasks and as these new ways are tested new products are developed. For a long time these new ideas were adopted very slowly. A look through old rope manuals shows the same methods being taught over and over again. Often these were not very good ideas when they were first thought of and over time they haven’t gotten better. Modern manuals have started to show radically new ways to perform these same tasks. Safety and technology improvement have driven most of these changes.

Training costs are another major concern. If a student is trained on equipment but can’t remember how to use it after the training is finished, the equipment has no use. Ideally a piece of equipment should be easy to learn and be simple enough for the student to remember how to use even when the student hasn’t practiced for a long time.

Accidents can happen at any time and probably are not going to happen immediately after a student completes training. Days, weeks, or months might go by and the student needs to remain proficient. Simple inexpensive equipment can sometimes be purchased that can do this but not always. It depends on the task that needs to be accomplished. A student can not expect to remain proficient with overly complicated equipment without extensive hands on time. This can come through operations or practice. If this is safety gear that is not used daily, the hands on time must come through practice and unfortunately most organizations are not able to devote this amount of time to training.

Traditional RPM (Rack, Pulley, Mariner's Hitch) Rope System

The CMC Multi-Purpose Device (MPD) combines a high-efficiency pulley, an integral rope-grab mechanism, a lowering device, and belay system into a simple to use piece of equipment. This allows a student to quickly and easily learn to lower, raise, and perform a safety backup without stopping to switch or replace hardware. The student can be taught in minutes and be able to easily remember how to perform these tasks after the training is finished.

The alternative to an MPD is a mix of equipment including a brake bar rack, pulleys, anchor plates, carabiners, prusik cord and load release strap. The time to teach the student to effectively use these items is much longer and most students will forget very shortly after the training is finished. This requires additional frequent refresher training compared with the MPD. These costs quickly become much more than if the proper equipment was purchased at first.

All equipment requires proper initial training and proper refresher training. Even with the best equipment this is still true. Finding the balance between the cost of the equipment, the initial training, and the refresher training is key. There are now many good products on the market and more are developed all the time. Please contact us so we can help you decide what is the best solution for your investment.

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Carabiner Wrap, Munter Hitch and Super Munter

It is common for people to play a game called, "What If". It usually starts with someone asking, "What if I don't have a Figure 8" or another piece of equipment. There are some critical pieces of equipment, such as a rope, but many other items can be improvised. In Thailand, it is still common to use a 'Carabiner Wrap' for rappels. To do this, a rope is wraped once or twice around a carabiner. This provides some friction to allow someone to rappel down.

Unfortunately there is very little friction added and many accidents happen because of rappelling this way. There are many videos on youtube taken in Thailand and other countries of serious injuries caused by this.

A safer method is to use a 'Munter Hitch'. This is a special type of knot where the rope passes against itself and creates additional friction. This can be used to safely rappel or lower a person. The rope is not damange since the rope is always moving. The rope doesn't cross the same point. One of the problems with the Munter Hitch is it twists the rope. If on a long rappel or multiple people rappel this way, the rope can turn into a tangled mess.

An even better option is the 'Super Munter Hitch'. This is Munter Hitch where the rope passes around itself a second time. This provides a lot more friction than a normal Munter Hitch. There is enough friction to safely lower two or three people at the same time. That is impossible with a normal Carabiner Wrap. Another benefit of the Super Munter Hitch is since the rope passes back over itself a second time, the rope does not get tangled.

Both the Munter Hitch and Super Munter Hitch require an HMS 'pear shape' carabiner such as the Rock Exotica Pirate Carabiner to work properly. A standard 'D' shape carabiner does not let the rope move freely.

Even the Munter Hitch and Super Munter Hitch are easy to tie and use, they still require proper training. Please contact us for more information.

 

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