Broken chair spine

Broken chair spine

We had just repaired the upholstery of my computer chair, when the steel tube forming the spine of the chair broke off. The single steel tube was not able to stand the lateral force of a person leaning against it.

Broken chair spine temporarily repaired by attaching a piece of wood and tied with string struts

Broken chair spine temporarily repaired by attaching a piece of wood and tied with string struts

It did cost quite a lot of money to renew the upholstery, and I could not bring myself to throw the chair away without doing something about it.

Since I did not have any welding machine to weld back the broken pieces of the tube, my first attempt to repair was to insert a solid steel round bar into the stem of the tube, but quickly found that it could not solve the problem. The solid round bar simply bent.

chair spine strengthened with a piece of wood and tied with string struts

Chair spine strengthened with a piece of wood and tied with string struts

Later, I tried to bind the two broken tubes with wooden splints, much like how we set broken bones together. I knew also that the lateral force of a person resting on the back would act in a certain direction, so I fashioned the “wooden splint” also to act in that direction. In order to secure the wooden piece to the steel tube, I had to drill some holes on the latter and secure the assembly by screws.

But that alone did not do the trick. I still had to tie the vertical steel rod spine of the chair with strings to take up the load, much like how the mast of a sailing yacht is supported against the force of the wind on its sails. But adding the strings made the chair look ugly. Furthermore, the arrangement was not entirely reliable, as the strings tend to stretch over time and became slack, thus defeating its function.

After I bought a simple welding machine for a different project (fabricating a tent canopy), I just butt welded the broken pieces together with the additional welding of a steel strip spine at the periphery. This solved the problem at that time and it lasted for quite some time.

Then trouble sets in again. The holes that I had previously drilled for attaching the wooden splint made the steel weak at these points. After many months of using the chair, fractures developed, starting from one of the holes in the highly stressed zone and quickly extended. The chair could not be used and had to be kept away, until I could find some time to work at it again.

When I had some steel rods left over from my other projects (garden shelves support stands), I worked out a design to strengthen the bent tube area of the chair backrest. This was my problem area and was highly stressed.

Increasing the 2nd moment of inertia for the backrest structure of the chair by welding struts and edges

Increasing the 2nd moment of inertia for the backrest structure of the chair by welding struts and edges

The idea was to increase the 2nd Moment of Inertia using the 8 mm diameter steel rods. By themselves, the steel rods were not very strong and would not be able to stand the force exerted by somebody’s back leaning against it, but by fabricating them wider apart with struts in between, they became strong. I had made them a good 120 mm apart at its widest part.

Since I had free rein to design, I fashioned it such that the backrest was located at the curve area of my spine, so that I could sit upright and have a good posture.

The chair seemed to be working fine. Although strong against the backward lateral direction, the backrest was still flexible and springy because it was not solid, but consisted of thin steel rods. Much will depend on the quality of the welds. I hope they could take the strain, although I am not too worried if they did fail some day. It will just be a matter of re-welding them again.

A coat of white paint made the structure look nicer.

Some new development …

broken strut

broken strut

After a few months, one of the struts of the backrest did indeed broke. As usual, I analysed the root cause. What I suspected was that the particular single weld joint was not sufficient. And there were some movement of the structure due to the top portion of the structure not being welded at all.

Additional struts on backrest frame

Additional struts on backrest frame

To rectify it, my corrective/preventive actions were to add in more struts. This time, I spent more time doing the welding in order to create more stronger weld joints not only at the vicinity of the broken joint, but also immediately behind the backrest. The latter was to prevent movement of the structure. Some cutting and re-welding was carried out.

Hopefully, this time the structure could last longer.