Posts Tagged ‘square steel tubing’

Wheelchair: Arms and Tie-Downs

Wednesday, July 29th, ©2009 Marcus Brooks
Wheelchair Arm Initial Configuration

Wheelchair Arm Initial Configuration

To make my power wheelchair’s arms as sturdy as possible, I built the seat frame’s front uprights and top crosspiece almost directly above the drive axle. The crosspiece is 1-inch, 11-gauge square steel tubing, extending the width of the seat to provide mounting points for the arms.

For each arm mount I built an “L” bracket with a 1-inch, 11-gauge upright and a 3/4-inch, 14-gauge horizontal piece. The 3/4-inch piece slides into the frame’s top crosspiece. The arm itself is a “T” with a 1-inch top and 3/4-inch upright, which slides down into the “L” bracket’s upright.

To secure the arm pieces temporarily, I drilled and tapped holes to accept #10-32 set screws. This allowed me to adjust the arm height and offset until I was happy with the arm positions.

To mount the joystick controller, I used #10-32 screws to mount a piece of 3/4-inch steel tube extending from the back of the controller box. This slides into the front of the right chair arm, where it is held in place by a #10-32 set screw. The screw has a knurled top so it can be tightened or loosened by hand.

The tubing I used nests together neatly, but the fit is not tight. Also, #10-32 set screws are really too small for the lower joints, which loosened quickly and began to wobble. To correct this, I drilled 5/16-inch bolt holes straight through each joint, front-to-back. I also slotted the outer tube sides, so the tube can flex and clamp the inner tube securely when the bolt is tightened.

Arm "L" Bracket and Tie Down

Arm "L" Bracket and Front Tie Down

Each slot is terminated by a somewhat wider hole (drilled before cutting the slot). In theory this hole distributes stress around the end of the slot, perhaps preventing a crack.

For this project, I standardized on 5/16-inch as my “large” fastener size. That is the size bolt used in the wheels I selected, but also its 1/2-inch head is the largest that fits a 4-inch crescent wrench (handy for field repairs).

Back Tie Down

Back Tie Down

I drilled the upright clamp holes a bit larger to take 3/8-inch eye bolts. I selected this as the best size for tie-down loops, to make them easy for the driver to use when I ride in a bus or taxi.

For the back tie-down loops, I likewise replaced one 5/16-inch caster mounting bolt on each side with a 3/8-inch eye bolt.