A monkey wrench is a type of size-adjustable hand wrench that was used heavily during the 19th century. Since then, this wrench style has lost ground to the more compact crescent wrench and the more easily-gripped pipe wrench. As use of the tool fell out of favor, the term began to apply to its replacements and size-adjusting wrenches. The original monkey wrench was used for many tasks, from fitting pipes to industrial manufacturing.
The design of a monkey wrench was very simple. A solid metal piece makes up the handle, slide and top jaw of the wrench. A screw sits on the front of the device and is partially screwed into the lower jaw. The lower jaw slides up and down the slide portion of the handle when the screw rotates. This design allows the user to adjust the size of the jaws, which open at a right angle to the handle, by rotating the screw with his thumb.
When the wrench is constructed, all of the parts are fitted together permanently. The majority of the handle is cast or welded together, with the exception of the top jaw. The screw sits on top of a metal post in the handle; this allows it to move freely, but not come off. The lower jaw is slid onto the slide, where the screw is secured into an opening in the jaw’s lower side. Then, the top jaw is welded onto the handle, preventing the wrench from ever coming apart.
The monkey wrench rapidly lost ground to other tools during the 20th century. This tool was directly replaced by the crescent wrench. The crescent wrench has a much smaller head and thinner construction. This allows it to fit into smaller areas but still perform the same tasks.
The design of the monkey wrench was taken by the pipe wrench. The main difference between these two tools is in the jaws. A monkey wrench has smooth jaws, like a crescent wrench, which were used for gripping nuts. A pipe wrench has grooved jaws, giving it the grip to hold onto pipes. This small difference makes the basic design two separate tools.
There is some debate over the origin of the term monkey wrench. One common theory is that it is a derivative of the name of its original creator, Charles Moncky. Other people state that it is a reference to the way the jaws come together; they look like a common toy monkey.