Step 1
Multiply the amount of weight that you feel comfortable lifting by two. This is the amount of weight that the pulley system will allow you to lift for the same effort. This is the minimum amount of weight that the hardware of your pulley system must be able to handle.
Step 2
Weigh one of the bundles of hay. Divide this figure into the weight capacity you decided on for your pulley system. The result is the number of bales of hay that your system will lift.
Step 3
Stack the number of bales you determined in Step 2 as closely together as you can. Do not stack them more than two high. Measure the length and width of this stack.
Step 4
Cut a piece of plywood to the measurements from Step 3. Weigh it. Subtract this weight from the weight of the maximum number of bales you calculated previously. Divide this number into the weight capacity of your equipment. The result is the number of bales that your system will actually be able to lift with the weight of the platform accounted for.
Step 5
Examine the pulleys, hooks and rope that you'll need for the system. Make sure that the rope is not too big for the pulley channels, and that it can fit through the eye hooks. Check that the pulley hooks can fit into the eye hooks. If you need to use a smaller rope or different pulleys, recalculate the numbers using the new equipment’s weight capacity.
Step 6
Assemble the system by screwing an eye hook into the rafter of the barn. Attach one pulley to this eye hook. Screw the other eye hook into the rafter about 1 foot away, then tie one end of the rope to that hook. Thread the rope through the second pulley and back up through the first pulley. Drill four holes, one in each of the corners of the wood platform, approximately 2 inches from the edges.
Cut two 10-foot lengths of rope. Make a handle out of one by tying a knot in its end and threading it through one hole. Push it through either of the holes adjacent to it, and knot that end so the rope can't slip out. Make the other 10-foot rope into a handle on the opposite side from the first. Pull the ropes to the middle of the platform so the hook on the second pulley can catch them both. If the platform is so large that they cannot both reach the second hook, then make larger handles.