Pulleys are amazing tools that we’ve been using as long as we’ve had ropes and pulley blocks. One of our newer models is the poleas trapezoidales. Unlike a round, or spherical, pulley block, it takes up a lot less space and can transmit more force because it pulls straight down on the rope instead of around a pivot point.
How it works
A trapezoidal pulley (also known as a Trapezoid Beloit Pulley) is simply a rectangular block with a lower block attached to it. That lower block can be made from stone, metal or plastic. The principle of operation is rather simple: as the rope travels through the pulleys, it is pulled straight down by friction on the blocks every 10 degrees of travel away from the parallel (flat rope path).
Why We Use Them
This model takes up half the space of a spherical pulley block and can transmit twice the force or more. Because of its shape, it will take up less room when transporting or storing.
Making the Pulley
The trapezoidal pulley is a simple idea, but a bit difficult to make, so we’ll start with an easy one: wood. Start by cutting a piece of wood into rectangles, similar to a large wooden ruler. Now, measure the middle of one of the long sides (the side that will be parallel to the rope). Make a mark there and measure out 2 inches. From that point, measure out another 6 inches on both sides. You should now have four points in a 4 by 6 inch rectangle. Drill holes through these four points, and then drill holes that are 2 inches apart in both directions starting at these 4 points.
Building the Pulley Carriage
We’ll need a way to hold the pulleys in place, so we’ll build a small carriage. Start by cutting another rectangular piece of wood that is 6 inches wide and 12 inches long. Let one end be 1.25 inches longer than the other so that we can attach it to our rope carriers at one end and to the trapezoidal pulley system at the other end.
Attaching the Carriage and Pulley
Let’s attach a rope carrier to each end of the carriage. At one end, cut a 2 inch piece of the same wood we used for the trapezoidal pulley system. Mark off half an inch on each side and drill a hole in each side. Attach this piece to one end of the carriage.