Posts Tagged ‘sprocket’

Wheelchair: Jumping Jack Shaft

Monday, January 10th, ©2011 Marcus Brooks

Some weeks ago I was careless enough… (I use that phrase too often) …to catch a length of gauze bandage in my wheelchair‘s right-hand drive sprocket. I heard a loud crack as the chair ground to a halt, and while hacking the gauze out I noticed some jagged shards of steel lying on the floor. This was not a good sign.

Jack shaft with trashed bearing. Note intact inner bearing, ball cage clinging to remaining outer race.

Jack shaft with trashed bearing. Note intact inner bearing; empty outer ball race/cage on shaft.

Apparently the weakest part of my chair’s drive train is the jack shaft ball race, not the chain as I’d supposed. Fortunately what was left of the cracked bearing still worked, so I just ordered a couple of replacements and more or less forgot the chair was wounded; even after the replacements arrived.

My quietude was shaken some days later when my brother found a loose bearing ball in the kitchen. I resolved to replace the bearing before an upcoming appointment, but things happen. I ended up leaving for the appointment on a chair of dubious safety.

Repaired jack shaft. Left to right: #25 chain/sprocket (from motor), inner bearing and hanger, collar (holds bearings in place), outer bearing and hanger, #35 chain/sprocket (to wheel).

Repaired jack shaft. Left to right: #25 sprocket/chain (from motor), inner bearing/hanger, collar (holds bearings and shaft in place), outer bearing/hanger, #35 sprocket/chain (to wheel).

The day’s bus rides included maybe half a mile rolling to and from stops, with hills. As I set out on the return trip, my drive chain started to pop and buck alarmingly. I spotted a steel ball in the parking lot. One of mine, lost on the way out? I suspected as much; also that they were all gone, or soon would be.

As I descended the bus ramp at the last stop, the chair bucked so badly I felt sure it wouldn’t make it home. I decided to call a ramp-equipped cab, but it was early afternoon. Three Austin cab companies assured me they had such cabs on duty, but not available. One eventually rolled up an hour later. (Three blocks cost me $5, including tip.)

I got home and halfway through the living room before the inner drive chain finally unshipped. My brother Randy pushed me the last few feet to my bedroom so I could get off the chair and start its disassembly. The pictures tell that story.

I was extremely lucky that day. One danger of my chair’s design is that a wheel rolls freely if its drive chain comes off. If that had happened on a ramp as I got on or off a bus… ouch. All considered, keeping that appointment was one of the stupidest things I’ve done lately.

Left: Intact Bearing; Right: Remaining Bits of Busted Bearing

Left: Intact jackshaft bearing; Right: Remaining/found bits of busted bearing