By Steve Garrett
The origins of the winch can be traced back to the most ancient records of man, and the mechanical principles governing its operation are among the most fundamental engineering concepts on Earth. Yet despite this humble provenance, or maybe because of it, the winch has become one of the most prevalent and vital tools of the modern age. I wanted to break it down a bit and look at the origins of the winch and its cousins the windlass and the capstan, discuss the principles on which they function, recount their development throughout history, and identify their continued purpose in the modern world.

Lets look at the Technology

A winch allows for the safe tensioning and storage of a length of line, rope, or cable around a horizontal drum or barrel. The line is attached to a powered drum, which can be rotated to draw in the cable, thus taking up the slack and tensioning the cable to hold some type of load. The winch is usually designed with a pawl and ratchet wheel or a clutch to allow for holding the tension on the cable without slipping. The drum can also be rotated to let cable out, thus slacking the line and releasing tension on the load. The winch is often fitted with a braking mechanism to allow controlled release of the tensioning force.

A windlass is like a winch in its design and operation, with the exception windlasses are often designed with the free end or trailing end of the rope attached to a fixed mounting point separate from the drum or attached to a counter weight of some kind, these are still used for ship anchors. Capstans, another technology in this family, operate in much the same way as winches, but use a vertically mounted drum that can be powered by either man or machine. Capstans also differentiate from winches and windlasses in that they do not store the excess line on the drum. Rather, a capstan will either store excess line in a separate container of some kind or will use a continuous 'messenger cable' that is attached to the loaded line by 'nippers'. The nippers are attached and removed from the lines as needed to allow the messenger line to draw in the loaded line, and then feed it into a hold while the capstan itself simply cycles the messenger line around in a continuous loop. This provides a safety benefit because if the loaded line were to suddenly surge backwards, the nippers would easily break and allow the line to 'free spool'. Alternately, the same scenario could result in severe equipment damage and personnel injury or loss of life if the loaded line were feed directly to the capstan.

All three of these tools operate on the principles of the Euler-Eytelwein equation.

"The Euler-Eytelwein equation relates the tension of the two ends of the rope: T2 = T1 eu0, where T2 is the tension in the rope due to the load it's supporting, T1 is the tension necessary to hold the load without slipping, u is the coefficient of friction between the rope and the pipe, and 0 is the total angle (measured in radians) made by all the windings of the rope (one full winding is 2 radians). The tension force increases exponentially with the coefficient of friction and the number of turns around the pipe."

As the Harvard University Lecture Demonstration excerpt above shows, as you increase the number of times you wrap the load line around the capstan, the force required to hold the tension on the line (or 'braking force') decreases exponentially. So, in theory, with the properly rated line and capstan, and enough wraps on the capstan, an average adult person could easily hold the load of two fully-loaded cruise liners. As previously stated, winches and windlasses differ from capstans in that they store the excess line on the drum. Considering the Euler-Eytelwein equation, this differentiation becomes very important for the proper understanding of how a winch or windlass is loaded as the line is let out. For example, a winch that is rated at 8,000lbs of pulling force is only capable of applying that force while there is one layer of line wrapped around the drum. When a second layer is added, the winches maximum pulling capacity is decreased due to the physics of having an equivalent larger drum diameter. This occurs again for each layer of cable wrapped on the drum (Schoenfelder, 2015). These concepts are important to understand when operating a winch, especially with loads close to the winches rated capacity.

A bit of winch history:

Winches, windlasses, and capstans are not new technology by any stretch of the imagination. They are so commonly employed in the daily execution of life's functions that they have become nearly invisible, but this was not always the case. As with most technologies we take for granted today, there was once a time when the winch was a wonder of engineering and innovation. In one of the earliest historical works, The Histories (440 BC), a Greek historian named Herodotus recounted the rise of the Persian Empire and the events leading to the Greco-Persian Wars. In addition to the politics and social posturing of the era, Herodotus also recorded the use of many technologies including a 'wooden windlass' used to tension ropes suspending a bridge across the river Hellespont. This is widely considered to be the first recorded evidence of winching technology. Of note in Herodotus' account is that he related the implementation of the winch as a matter of course, as if it were a common tool even in 440 BC, which suggests that it was being used prior to Herodotus' account. The specific invention date of the winch may be lost to time, but by the publishing of the Ancient Grecian literary work Mechanical Problems (Winter, p.18) in the fourth century BC, winch hoists are common enough to prompt philosophical pondering of the physical principles of their operation.

It is difficult to accurately account for each change in winch, windlass, or capstan design throughout recorded history. Their basic design features and constraints have remained unchanged for thousands of years. What has changed are the materials used, the modalities of powering their operation, and the degree of engineering involved. These iterative changes have resulted in modern versions of these tools that are lighter, smaller, faster, more powerful, and more intelligent than ever before. Once crafted from wood and iron, winch drums now use steel, aluminum, and even titanium in some specialized applications. Instead of human powered direct drive, modern drive options include electric, hydraulic, and pneumatic, each available with either a planetary, worm, or spur gearbox that trades speed for higher torque (and thus more loading capacity). Today's winches also have options for a full range of instrumentation that allows the operator to monitor virtually any aspect of the winch, including length of deployed cable, cable remaining on the drum, tension on the load, drum speed, power consumption, and more. In Herodotus' time, winches were used only in construction and seafaring applications. Look around today and you will see winch technology everywhere. Cranes, tow trucks, commercial shipping vessels, sailing vessels, auto-repair shops, off-road recreational vehicles, helicopters, electrical power generation, rock climbing, military vehicles, farming industry, etc., etc. Anywhere a person needs to lift or move something heavy, you're likely not too far from a winch.