18 April, 2009

Buses and claustrophobia

I had to go on a service call Thursday evening to repair a bus that was leaning severely to the right. No mechanic in his right mind likes working on buses. Just look at them. They're so low to the ground that a person can barely fit under it, if at all. Throw in an air suspension problem and the bus is even closer to the ground. I was the only mechanic available in the shop at that time and that meant I got stuck with this service call. We spoke with the owner of the coach and he told us what needed to be done. Apparently they've had this same problem with most of their coaches, so this guy new the fix for it. There's a leveling valve for each side of the bus which, in a primitive sort of way, gives the bus something like independent suspension. Most trucks have only one leveling valve to control all air bags (or air springs if you prefer) and it's adequate for the job. A bus on the other hand, is all about ride quality.

A leveling valve, properly called a "ride height control valve" or RHCV, is a very simple thing. The valve has (mounted to the vehicle's chassis), at the minimum, three ports. One port delivers an air supply to the valve, a second port will supply air to the bags, and the third port is an exhaust port. Sticking out of the valve will also be a shaft to which a lever is attached. Moving this lever moves the internal parts of the valve and either lets air flow to the bags, or exhausts air from the bags. There's also a neutral position that neither supplies air nor exhausts air. So, we have a lever with one end connected to the valve, but what about the other end? The other end is connected to a linkage rod. This linkage rod is connected to whatever axle the leveling valve controls ride height for. So how does it all come together in operation? It's very, very simple. Air bags are filled with, no surprise, air. Air can be compressed and when a load on a vehicle is increased, the air inside the bags gets compressed. As the air in the bags compresses, the chassis of the vehicle starts to lower towards the ground. Now since an axle (axles aren't considered a "sprung" component) will remain stationary and the linkage rod won't flex, as the chassis lowers the linkage rod causes the valve's lever to move, in turn the lever operates the valve. As the valve shaft turns it uncovers (in this case) the supply port allowing air to flow into the bags. Not surprising at all, as air flows to the bags, the air pressure inside the bags increases. As the pressure increases, the bags will raise the vehicle back up to a pre-determined chassis ride height (set with the linkage rod.) Are you confused a little bit? If you are, it's okay. To understand the principles of how a leveling valve works, just go to your bathroom. Seriously. Pull the lid off of the toilet's tank and look inside. Think of the valve inside the tank as a leveling valve and think of the float as the linkage rod. The water in the tank is like the air supply to the leveling valve and think of the toilet bowl as the air bag. Now, flush the toilet. Notice that the float drops as water in the tank drains away. When the float drops, water flows through the valve into the tank. When the tank reaches the full mark, the float shuts off the valve, stopping water flow into the tank. That's how a leveling valve works :) Got it? Good.

Anyway, this bus I was out fixing had a common problem. The owner told me to disconnect the air lines from the valve and look for debris blocking the ports. This would be a simple job if it were a truck, but it was a bus with all of the bags on the right side deflated. It looked like the leaning tower of Pisa, believe me. Guess where the leveling valves were? In between the drive axle and tag axle where there was a lot of room to work? Hell no! They were in front of the drive axle, above some really big air tanks. I thought I'd crawl in between the drive axle and tag axle (the tag is to the rear of the drive BTW) wiggle through some torque rods and be able to get at the offending valve. Nope, didn't happen. I could see the valve just fine, but I couldn't reach it. Hmmm, now what? I called the owner again and asked him how I could get at this valve. His suggestion was obvious, but I hadn't thought of it. Buses, usually, can be raised and lowered manually if the need arises. Sometimes a bus needs some extra ground clearance to clear, oh say, a steep driveway entrance. The driver can operate a valve which bypasses the leveling valve and inflate the bags to maximum capacity. It's not really safe to go down the road like that, but it's okay for low speed maneuvering. Back to the story. The driver raises the bus as high as it will go and as I looked in from the top of the right drive tires, I could see the leveling valve I needed to fix. There was one small problem though. With the bus raised I could fit between the top of the drive tires and the bottom of the bus's body, but disconnecting the leveling valve lines would cause the bags to deflate. If the bags deflated, the bus would lower. Crushing me like a rotten tomato. So, I had to find a way to do this job without ending up dead. I found that I could stack some blocks between the drive axle and the chassis. After doing so, I had to driver lower the bus. Of course with the blocks in place, the bus didn't go down, but the air bags deflated and I still had the clearance I needed to crawl in. Now I could get a wrench on the air line fittings. I couldn't, however, get my hand on the air line fittings. Not a big deal when removing the air lines, but I had to be able to get my hand on them to reconnect the lines. Twenty bolts later I had the fender flare removed and could put my hand on the leveling valve. After disconnecting the lines I used a blow gun to blast the leveling valve's ports and, sure enough, some big chunks of debris came shooting out. Not sure what it was, but it was white so I'm thinking it was pipe thread sealant. I reconnected the lines to the valve and had the driver start the engine to build air. After we had enough air pressure, the driver raised the bus and I removed the blocks I had put in. With the blocks removed, the driver lowered the bus and put it into "travel" mode. Woohoo!!! All of the bags inflated as they were supposed to. I bolted the fender flare back on and had the driver drive around the parking lot to make sure the leveling valve was still functioning. It did and that made me very happy. The service call ended on a high note because the driver paid in good 'ol cash! No comm check systems to deal with, no credit card hassles... cash is always nice. After a handshake and a "Have a safe trip" I was back on the road, heading for the shop.

I started to shake a little bit on the drive back because my fear started coming to the front of my mind. I have mild claustrophobia and being under a bus like I was scares me. Mix in the possibility of being crushed and things border on sheer terror for me. But the job isn't going to take care of itself. Trucks don't bother me because they're an "open air" environment you might say. Not buses. When I face situations like this I would rather be doing just about anything else, but somehow I manage to put my fear aside and scrape up enough courage to get the job done. I'll just have to deal with the fear later. Maybe one of these days I'll have a nice office job...

14 April, 2009

Hand cut dovetails

I spend a lot of free time on woodworking projects of all sorts. I find it relaxing (most of the time) and it's a great way to de-stress after a rough day of fixing trucks. Last summer I started to do more work with hand tools and it has been quite a learning process. I've been concentrating on getting better at cutting dovetails and, for the most part, I'm improving. Slowly. If you go to YouTube and watch videos of a pro cutting dovetails or read about it in a woodworking magazine, it seems to be a very simple process. It is an easy process, but the execution is much more difficult. You'd think that something as simple as sawing to a line wouldn't be very difficult but it is. Another thing that can make dovetails frustrating is not knowing what a sharp tool is. Cutting dovetails with hand tools involves many different skills, and those skills don't come without some practice. For example, the previously mentioned sawing to a line. I thought it would be a simple matter of buying a backsaw, marking a line and then following it. As I found out, there's a lot to learn. It took a lot of practice to finally see that I consistently deviate from the line in one particular way. After more practice I learned how to prevent that deviation. I'm still not great at sawing, but I'm a lot better than I was a year ago. Oh, the saw itself plays a key role. I've found that a Japanese style pull saw (without a back) works well for me versus a traditional western style back saw. There's one style of western saw I've yet to try, but money is short and a new saw isn't in the cards at the moment. In the photo (it's a tailboard) you'll see the tails marked out. The shaded areas are the waste which needs to be removed. I use the pull saw to make the vertical cuts and then use a coping saw to make the horizontal cuts. If you are like me, you would probably mark a line and saw right down that line. That method doesn't make for nice fitting dovetails. The vertical cuts are made on the waste side of the lines and the horizontal cuts are done in a similar fashion, but I leave a little more waste. After the waste piece is cut free, I use a chisel to pare the wood down to the horizontal line (it's called a base line I believe.) Chiseling, there's another skill that didn't come naturally. The biggest problem with chiseling is having a properly sharpened tool. When I bought my first hand plane (a Record smoothing plane) and first set of chisels, I had no way to sharpen them. Hell, I thought they came out of the box ready to go. They don't, trust me. A trip to the "Big Orange Box" for a sharpening stone and I thought I had it made. Nope, not yet. After digging through the stacks of old woodworking magazines, I learned that the back of a chisel needs to be flattened before working on the bevel. The same goes for plane irons. After a few hours flattening chisels and plane irons, my arms felt like Jell-O and my fingers were raw. But, I had flat tools. Next it was on to the bevel. I used the side sharpening method on the bevels because it was easier for me to hold the tool in the proper way. Two hours later I had what I thought were some razor sharp chisels. Any professional woodworker would have declared them dull. I didn't know it at the time, but the cheap sharpening stone I bought (the only one I could find locally) was pretty shitty. I also had a hard time keeping the chisels in position while sharpening them. Mail order to the rescue! I bought a honing guide which eliminated my problems with holding the chisels, but I still had a crappy stone. The chisels were sharper than before, but not much. Last weekend I finally decided to give the "Scary Sharp" method a try and wish I would've done it years ago. It is nothing more than putting various grits of sandpaper onto a reliably flat surface (i.e. plate glass, marble, granite etc.) with spray adhesive. You start with the coarse grit and then work your way through the grits in succession until you reach the final one (2000 grit in my case). The combination of the honing guide, flat glass and the many grits of sanpaper yielded a mirror surface on the first chisel I sharpened. I gave that first chisel the age-old test of trying shave some hair off my arm. Shave it did, and cleanly at that. My plane has also been a joy to use. Right now, all of the skills I've been working on are beginning to come together and are yielding better work. I put the dovetailed box together the other day and it looks very good (for me that is) with only a couple of really bad gaps. Those gaps are because I cut on the wrong side of the line on a pin board. Stupid mistake, but I learned from it. I'm looking forward to getting the box glued together and using my razor sharp plane to trim the joints flush. That will have to wait. I got sick of my workbench racking and squeaking so I disassembled it (I didn't build it) and decided to renovate it with mortise and tenon joinery. I plan on building a new bench in the near future, but reworking my old "squeaker" is turning out to be a good learning experience. But, that's a whole other story.