Keyboard tray annoyances

Ladies and gentlemen, finally, my second post is here. This time, I would like to highlight yet another annoyance I have regarding everyday products, namely the keyboard tray from a typical computer desk sold in Staples, Officemax, and the like. The keyboard tray is typically mounted on a linear ball bearing slide. The housing of the linear slide is fixed to the main desk platform with a simple right angle bracket, shown in the first figure.

Now the keyboard tray works just fine if you push it in and pull it out in the middle of the drawer. But if you push/pull at the left or right-most location, it will bind. This happens with my last two computer desk. They all have the same design. This binding is due to a loading condition that is different from the specified function. When you apply the load at the center of the tray, the loading condition is a pure force or translation forward. But when the load is applied on the extreme left or right points on the tray, it creates a translation AND rotation, as illustrated below.

The culprit of the problem is in the brackets that hold the linear slide frame. These brackets are stiff in the direction of the pure forward translation, but not very stiff when subject to side load. The induced rotation causes an amplified side load on the brackets. In fact, the wider the tray, the greater is the multiplication factor of this side load due to rotation of the tray. Because the bracket deforms as the tray rotates, this creates binding in the ball bearings along the linear slide. This is not anticipated by the design. One easy solution is to put gussets on the bracket. Another solution is to have side walls that coincide at the location of the slides so that the slides rest on hefty side walls or main frames of the desk. The pictures below illustrate alternate design that solves this problem inherently in the design without having to reinforce the brackets.

It is very important for a design to be robust, which means that its performance does not decrease as the assumed factors deviate from normal parameters. This parameter includes specifications such as loading conditions. If a simple table is designed to support your desktop items which weigh at most 10 lbs, why do you think it is designed for 300 lbs capacity? That’s right; somebody is going to sit on the table. People use textbooks as a step ladder, and who knows what people do with duct tapes. The point is, we always design things with safety factors as a margin. But safety factor is often specified for one particular loading condition. Here, the performance drops immediately to the point of non-functional as the assumption (tray is pushed/pulled at the center location) is violated. This is an important design principle – robustness.