One of my hobbies is racing motocross on vintage motorcycles. On one of my favorite motorcycles is a 1971 CZ 400, which was manufactured in Czechoslovakia. An important part of the off season maintenance on this particular bike is to clean, lubricate, and readjust the swing arm pivot. Without going in to a lot of detail, the swing arm fits between frame members and is held in place by a steel pin that passes though holes in the frame and swing arm. Tension is adjusted by tightening bolts that thread into the pin, which compresses everything together via a pair of cone washers. Figure 1 shows the basic configuration. This design was in use on this motorcycle from the early 1960s to the mid- 1970s.
Figure 1. Schematic of CZ swing arm setup.
As you can imagine, once the pin that passes through the assembly is taken out it is easy to disassemble the components so they can be maintained. Reassembly is not quite so simple, at least for one person, in large part because inserting the pin while positioning the swing arm and fitting the cone washers requires at least three hands. There are some tricks to make this easier, such as putting a dollop of grease on the cone washers to hold them in place and doing each side in series, but these are only marginally successful. The net result is that reassembly usually takes multiple tries as the cone washers drop off and fall on the work stand. While spending a few extra moments in the shop indulging in my hobby isn’t a terrible hardship for me, this is clearly not a design that had the maintainer in mind.
Jumping ahead four or five decades, I was recently asked to conduct a human engineering analysis of a new seat being designed for the military . This analysis was conducted from both the warfighter’s perspective and from the perspective of the maintainer, who happen to be the same person in the target user community. This seat is being designed to some demanding requirements including, but not limited to, weight, volume, adjustability, reliability, mean times to repair and replace, soldier survivability, and of course acquisition and life cycle costs. One of the requirements is that the seat back be removable and replaceable by a single maintainer in a very short period of time. Removal of the seat back is required to allow a maintainer to access other equipment more easily. Figure 2 shows the basic design of the attachment between the seat back and the seat base. Does this look familiar?
Figure 2. Schematic of military seat back-seat base junction.
Yes, the design being proposed for this new, state-of-the-art system is very similar to that found on my old motorcycle. One problem in the military seat design is the use of washers to eliminate lateral play between the seat base and seat back. Reinstalling these washers presents the maintainer with almost exactly the same problem as I find on the old motorcycle – how do you keep them in place while reinserting the pin that holds everything together with just two hands?
I tried to perform this operation in a nice warm, dry, well lit mock up of the military vehicle and indeed managed to drop the washers. Now imagine a military maintainer attempting to perform this task in a stressful environment, perhaps while wearing winter or other protective gloves. The outcome is likely to be dropped washers that aren’t found on the floor and consequently aren’t reinstalled. I should mention that in military systems of this type it is frowned upon to use a “dollop of grease” trick to hold the washers in place because the grease will collect dirt in the operational environment, becoming its own maintenance headache. It is also unacceptable to simply drop the washer on the floor and not recover it. There is no way of telling where the washer will end up and what the effect will be. For example, the lost washer could roll around on the floor and end up jamming the vehicle’s controls.
The bottom line is that by making a minor change in the design of the seat back that reduces the space between the seat’s base and back rest we can eliminate the need to use these washers, and alleviate the maintainability problems with the use of these washers in this venerable design. As a result of making the design change, one person can easily reinstall the seat back, the accuracy of the maintainers is increased (no washers are left out) and the time to replace the seat back after it has been removed is reduced, all of which are important metrics in the acceptability of the system. There are also cost implications. While improving the maintainability of a design to save a minute or two is of little consequence to a hobbyist working on a single machine, when hundreds or thousands of military maintainers perform a task repeatedly on their systems the cumulative time and cost adds up quickly. Now, if I could just redesign the old motorcycle I could spend a few more minutes riding.
Tom Sharkey is a senior scientist at MTI. He has over 25 years of experience providing human factors engineering support across a broad range of ground vehicle and aviation programs, both military and commercial. He is based in our Denver, CO office and can be reached at tsharkey [insert the at sign here] montereytechnologies.com.
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