Sunday, July 13, 2014

Step ladder with spring-loaded casters

A couple of months ago my sister commissioned me to build a step ladder tailor-made for her kitchen so she could reach the top shelves of the overhead cabinets. More importantly she wanted a way for her reckless 5-yr old to safely reach the cabinet (to get his juice bottle) without scaling the countertop and precariously making his way up (drumming in a list of don'ts just hasn't worked).

I didn't want to make just another run of the mill ladder. I wanted a sleek design that didn't look like just another a ladder. The original design did not include the 4 feet extending diagonally off the bottom of the ladder. But I had to add them because the ladder had the tendency to tip over whenever a person shifted their weight close to or over the railings. I implored and pleaded with physics and gravity, but they just wouldn't make an exception for this project. Much as I don't want those feet--they ruin the look--they're essential for safety.

I also wanted a ladder that both kids and adults could move with ease. My sister suggested putting two non-swiveling casters so that the ladder could be tilted on those casters and then wheeled into place like a hand truck. That was fine with adults but I was concerned--given the height and weight of the all-steel ladder--that that would result in accidents when my nephews started rolling the ladder. No, that option just wasn't kid-friendly. And so I decided to use spring-loaded casters.

The problem is, I couldn't find any hardware store selling such casters. Hence, I was forced to design one that I could, with my paltry fab skills, build on my own. I couldn't think of an easy way of putting the spring directly on the caster so I decided on using a chassis to bolt the casters on and then mounting the springs on half-threaded bolts to function as guide pins which would mate the chassis to the ladder.

The frame of the ladder including the feet is made from 1/16 x 1" square tubing; the steps, from 1/16 x 1 x 2" tubing; the chassis or "H-frame" for the casters, from 1" angle and 1/16 x 1" tubing. The ladder is 48" high, 17" wide and 14" deep, excluding the feet and caster system. The steps are 9" apart vertically, thus the topmost step is 27" from the ground.

I was able to purchase a spring about 20" in length and about half an inch in diameter. I did some crude tests with a weighing scale to measure its displacement versus force characteristic. For this particular spring it came out to about a quarter inch per kilogram. The ladder was around 13 kg so each of the four springs would be bearing a little over 3 kg. I eventually ended up cutting approximately 2-1/4" lengths of springs.

For the guide pins I used 3/8" x 3-1/2" galvanized hex head half-threaded bolts that have a 2-1/4 inch length that isn't threaded. These weren't ideal but the hardware didn't have any 3/8 bolt which had 2-1/2 to 2-3/4" of non threaded section.

Among the mostly black, shoddy-looking casters in the hardware, I found gray-colored rubber 2" swivel casters that really look good and whose color would match the finish of the ladder

As can be seen in the photo below I temporarily tack welded the H-frame the tubings which needed to be drilled so that the holes would register as precisely as I could make them.

For the majority of the welds I used MIG--am far more comfortable with that process. But I had a new TIG machine and so used it for just about all of the butt welds, except for the steps of the ladder which were all MIG welded--I was just too ill-confident and lazy to TIG weld them.

After grinding down all the non-fillet welds and cleaning the metal I painted it with a coat of red oxide primer. Sanded it afterwards in the hopes of getting a shinier finish with the topcoat. Mys sister's kitchen cabinets are in gray and orange so I painted the ladder likewise.

H-frame tack welded to the square tubing that will eventually be welded to the ladder. Temporarily joined together I could precisely register the holes that had to be drilled on both. When it was time to weld the tubing to the ladder I bolted (without the springs and using full threaded screws) the H-frame and the tubing together to keep the four holes aligned.

Te spring-loaded caster system. The two 1" square tubing pieces will eventually be welded to the ladder

Letting the paint dry completely before installing the caster system. Black rubber end caps are installed on the feet