Barbell Pronator - the Jackhammer

A simple pronation and supination trainer for the end of an Olympic barbell. The hub slides over the sleeve, a strap winds onto the drum, and you roll your wrist to lift the weight. Because the bar acts as the axle, your biceps stay out of it, and the resistance stays fairly even through the whole range of motion.

Each pound on the strap equals about 2.6 inch-pounds of twisting force. Small loads go a long way, so 5 lb is a good place to start.

Files

• Hub: slides over a standard 2 inch Olympic sleeve (51.7 mm bore)

• Handle: bolts to the hub

• Grip: knurled, 32 mm diameter

What you need

• 4x M6 x 25 mm socket head screws and 4x M6 nuts

• 1x M8-1.25 x 160 mm bolt and M8 lock nut (for the grip)

• About 4 ft of 1.5 inch webbing, a 1.5 inch cam buckle, and a carabiner

• About 750 g of PETG

• A weight pin to hold the weight plates

Printing
I printed mine in PETG with 40% gyroid infill and 4 walls on a 0.6 mm nozzle (use 5 walls on a 0.4 mm nozzle). No supports needed.

• Hub: flange down

• Handle: flange down

• Grip: standing on end, with a small brim

Assembly

1. Drop the M6 nuts into the pockets inside the drum, then bolt the handle to the hub. Snug is enough.

2. Put the grip between the handle uprights and run the M8 bolt through. Tighten it until the grip stops spinning.

3. Feed the webbing in one slot of the drum and out the other, then fasten the end back onto the strap with the cam buckle.

4. Wind the strap around the drum, and clip the weight on with a carabiner.

Use
Slide it onto the bar and let the weight hang. The direction you wind the strap decides whether it works pronation (palm down) or supination (palm up).

Check the bolts after the first few sessions, since PETG settles a little. Keep the load at 25 lb or less, and inspect the parts now and then for cracks.

How heavy does it feel?

The strap winds at about 2.6 inches from the center of the bar. Multiply the weight on the strap by 2.6 to get the twisting force in inch-pounds.