Friday, July 6, 2012

Repurposed Garage Door Opener

A while back, one of our craftsman 1/2 HP chain drive garage door openers stopped functioning properly. The doors stopped moving, even though the motor still ran. A quick inspection revealed that the sprocket on top of the unit sheared off the shaft. Not knowing any better (and being under a time crunch to get the door working), a new opener was bought and installed. However, it seemed like a waste to throw away the old unit, since everything else still functioned properly (the motor, electronics, etc.), and so I put it aside for future scavenging.

Recently, I came into some free time, and decided to automate the closing of my bedroom door. Plenty of times, when I'm all comfortable in my bed watching television, it gets noisy outside of the room; getting up to close the door, at the time, always seems like such an undertaking (and then you have to get comfortable all over again). I brainstormed through some concepts for a while, and decided to try to revive the old opener and re-purpose it for the task.

A quick search on Google revealed a replacement gear shaft assembly that included the broken sprocket for around $23 on Amazon ( I wish I had known earlier instead of paying for a new opener). When the parts arrived, the new gear shaft assembly was installed in a snap, and the unit was back to fully functioning condition in no time. After dis-assembly, I decided to look at the failed part to see what went wrong, and lo and behold, our old friend "planned obsolescence" showed its head. It seemed that the engineers at Sears purposely ground down a segment of the shaft in order to lower its endurance limit. For those not familiar, in materials science and mechanical engineering, a concept known as the "endurance limit" for a material determines the maximum cyclic stress that a material can experience and have infinite life. If that stress is exceeded, the material then has a finite life cycle (amount of stress cycles before fatigue failure). Since the tension of the chain is always pointing in the same direction, and the sprocket rotates, then the shaft experiences cycling loading.

The modification can best be seen in the picture below. Note that the ground down portion is completely hidden under a metal cap, and cannot be seen by the user (even if the unit is disassembled). Very sneaky.


Because of the metal cap that can only be removed once the shaft snaps, It is not clear if the new shaft also has this feature (I would guess yes, since it is a craftsman replacement part). Just for my own curiosity, I am going to run some calculations in the future and see if the stresses are indeed above the endurance limit.

In order to convert the linear motion of the garage door opener into something that will swing a door shut, several options were considered. What  made it more challenging was the fact that the door followed a path that traced an obtuse angle (see figure below).

 
I could not simply use the rotation of the sprocket itself, since it rotated way too fast. One option was to slow down that rotation using some bicycle gears, but it seemed too cumbersome and would need a lot of parts and work. another option was to use a rack and pinion system (rack on the trolley, pinion on a separate shaft that swings and closes the door). That option also required a lot work and could potentially take up too much space. The final design used simple linkages to push on the door, shown better in the illustration below:



The lengths of the linkages had to be chosen precisely in order to complete the required motion, while also avoiding the condition of "singularity," a point where the two linkages line up and theoretically requiring an infinite amount of force to move (in reality, it will yield the material and buckle the beams).

Surprisingly, I was able to build the whole mechanism using only parts found in a standard garage door opener kit and $5 worth of hardware from the hardware store. Since I re-used all of the old parts with the new motor that I purchased, all of the new parts that came with it (brackets, track, chain, etc.) were just laying around. This saved me a lot of money, work, and frustration, and made it that much more awesome when completed.

After the motor was back in working order, the track was shortened to only half of the original length, and consequently, so was the chain. The whole unit was reassembled and the travel was adjusted for the shortened track length using the built in adjustment screws. The trolley assembly was flipped 180 degrees to make the assembly more compact, and a bracket was installed at the end of the trolley to serve as a pivot for the upper linkage.

The kit came with two pieces of angle iron that had holes every 2 inches or so, to be used to hang the motor from the ceiling. I instead used those pieces to prototype the linkages (the holes also allowed for on the fly adjustment of the intersection point of the linkages). After the linkages were set and the travel fine tuned, the prototype was attached to the wall using the regular mounting brackets from the kit. The safety sensors were taped facing each other, or else the motor will not function (SEE NOTE BELOW), and the activation switch was wired up. Finally, a cover was made for the motor to make it look nicer.

NOTE: I AM NOT RESPONSIBLE FOR ANY HARM CAUSED BY NEGATING THE SAFETY MECHANISMS. THIS IS PURELY FOR INFORMATIVE PURPOSES ONLY. the amount of force produced by this machine is tremendous (mechanical advantage is no joke) and should be treated with respect. Do not attempt to replicate this unless you know what you are doing.

The finalized prototype is shown in the images and video below:










 
 
 Action video for both conditions: fully opened, and partially opened:




Overall, the project seemed to fulfill the goals I had set out. It was relatively cheap, was pretty compact and out of the way (the motor fit nicely underneath my desk), and only took about two days of work. In the future, the linkages will be replaced with stronger steel flat bar, since these bars are intended for tensile applications and not compressive forces. And also as a bonus, all of the parts can be found in a garage door opener kit and at your local hardware store.