Redesigning a commercial treadmill line
A curved commercial treadmill that kept coming back to the factory, redesigned so it could be built right the first time, shipped without damage and fixed in the field.
- Returnsfull-machine factory returns cut by more than 90%, as reported by the manufacturer
- Redesignabout 70% of the machine
- Shippingup to four machines on one pallet footprint
Where it started
This was before Atrick Design, while I was the product developer at a Connecticut contract manufacturer. The treadmill line was losing money to full machines coming back to the factory. I took it over, redesigned about 70% of the machine, and retrained the assemblers to build it. The approach was not to inspect harder, but to design the product so the floor could build it right the first time.
A smoother, quieter belt
Foam sat under the belt as a cushion. The fasteners crushed it, the belt could rack out of square into a parallelogram, and the foam together with the scalloped treads made the belt chatter. I redid how the treads and the belt are sandwiched together, with aluminum spacers in place of the foam under the fasteners, planned to be molded into the treads in a later revision. The treadmill ran a lot smoother. I also checked the tread slat loading in simulation.
Rollers, pulleys and the one-way bearing
Rollers. The shouldered rollers had two bearings sandwiched together, so the bolt holding each roller could not be fully tightened. Over time some rollers bound up and others worked loose. A nylon-insert lock nut behind a self-tapping bolt had been used to set them, which made the treadmill noisy to run on. I changed them to a single bearing with an interference fit: heat the roller to 250 °F (121 °C) for about ten minutes and the bearing drops right in, then holds for life once it cools. That took about 14 bearings and 28 circlips out of each machine, along with some bearing pressing.
Pulleys. The original pulleys were fitted very tightly onto their shafts and pinned through with a split pin, which made field repairs very hard, and they bound up the shaft bearings until they rattled and got noisy. The new pulleys sit on a cone seat with a square O-ring on each side, between the pulley and the bearing, to cushion the assembly and stop the rattle. The bearings can now be replaced in the field.
The one-way bearing. It was undersized and solidly mounted, and it seized. I upsized it and put it in a rubber mount so it can float, which also cushions the shock when someone pushes the treadmill backward.
Covers and fasteners
The side covers were drilled by hand, and the holes did not always line up. Bolts cross threaded into PEM nuts that had been tack welded in place, so nuts popped out and bolts snapped. I changed the covers to a tab-and-slot design so the laser cuts every hole. That took a little more care bending the top shape, but every cover came out the same.
The PEM nuts became hex rivnuts, which are easy to replace in production and in the field. An axial ball bearing, or even two washers and a bolt, is enough to install one. More on the fastener fix →
Handrails that ship in a small box
The welded handrails were bulky to ship and to mount. As I recall, a replacement went out in a box about 4 × 3 × 4 ft (about 1220 × 910 × 1220 mm) that usually ended up on a pallet. I changed them to a three-piece design. A replacement now ships in a box about 4 ft × 6 in × 3 in (about 1220 × 150 × 75 mm), and painted handrails nest together, 17 in the space 2 used to take.
A crate that stacks four high
The treadmill used to ship fully assembled on a pallet inside a big cardboard box, so anything that hit the box hit the machine. I designed a wooden crate that stacks up to four high on one pallet footprint. Gyms ordering machines in big batches could pay about a quarter of the shipping price, and the crate cut shipping damage too.
Fixes customers could install
Several of these fixes could go out to machines already in the field. Replacement rollers were sent to machines whose rollers had bound up or worked loose, the new pulleys let the bearings be changed in the field, and the rivnuts go in with simple tools.
A display that protects its battery
The console display had gone into production as an early Arduino-based prototype, with wires pushed into header pins. It drew a lot of power even when no one was running on the treadmill, its wiring caused problems, and it had no low-battery warning, so the sealed lead-acid battery could run down to about 6 V. Those batteries do not tolerate being discharged much below about 11.4 V, so batteries were failing and replacements were shipping out.
I designed a new board that keeps the same look from the front. It has static protection, an efficient switching regulator and a battery charge warning, plus a connection plate that doubles as a test point for the speed sensor signal and the battery level. The circuit was proven on a breadboard first, test code checked that every LED was addressed correctly, and the design was nearly ready for production when I left.
The power inlet, where the treadmill charges its battery or runs without one, started as a hand-soldered wire assembly with no fuse, and a shorted lead-acid battery can put out a lot of current. My quick fix was a small board with a self-resetting fuse. The planned replacement folds the inlet, the battery level check and the speed sensor check into one board with plug-in connections and test buttons, in place of a large wiring harness.
The result
After the redesign, the manufacturer reported full-machine factory returns down by more than 90%.