Well, here it is, fall. With summer over and the kids back in school, life is settling down to a familiar routine for most people.
The commercial tire business also is beginning to slow down a bit as temperatures cool, and this may give us some time to wonder about the imponderables of the world. Let's put aside questions such as, ``Why do clocks run clockwise'' or ``Where do houseflies go in the winter?'' to instead consider the more serious query: ``How do inflation gauges, impact wrenches, torque wrenches and tire jacks work?''
I know that question has bothered me, so I thought I'd pass along some interesting pieces of information that you can use at your company Christmas party to impress your boss and colleagues.
Healthy inflation gauges
Let's start with the ``stick'' inflation gauge. You've probably been wondering as I have: ``How the heck does that thing measure the pressure?'' ``Why doesn't the little scale blow out the end?'' and ``What's inside that thing anyway?''
Well, inside the tube that makes up the body of the pressure gauge is a small, tight-sealing piston. The inside of the tube is polished smooth and lubricated with a little oil. The piston is made of rubber so that it seals tightly against the tube, and the oil helps to improve the seal. There is a stop at the end of the gauge that holds the piston in the tube.
A spring runs the length of the tube between the piston and the stop, and this compressed spring pushes the piston toward the chuck end of the tube. Inside the spring is the calibrated rod or scale, which is not attached to the piston but rides on top of it.
The chuck end of the gauge has a rubber seal and a small fixed pin. The rubber seal presses against the lip of the valve stem to prevent air from leaking while you're taking the tire's pressure, and the pin depresses the valve pin in the valve stem to let air flow into the gauge. The air flows around the pin, goes through the hollow passage inside the chuck and rushes into the piston chamber. It then pushes the piston so that the pressure scale comes out while the spring pushes back in the opposite direction.
If you have a gauge that is designed for some maximum pressure, say 160 psi, the spring is calibrated so that 160 psi will move the piston to nearly the end of the tube, while 80 psi moves it half-way, and so on. When the pressure is released, the piston moves back towards the chuck end of the gauge, but the rod stays in its maximum position to allow you to read the pressure. And voila, you have found the tire is indeed flat!
Now that you know how gauges work, you can see the need to keep them clean and dry and not dent the tube. This can cause parts to stick, giving you inaccurate readings. Since most service gauges are not repairable, if your gauge is off by 5 psi or more, it's best to throw it away and get a new one.
Impact wrench checklist
Impact wrenches-like the pressure gauge-are operated with air, too. Impact wrenches work by using air to power a motor that turns a rotating weight that slams repeatedly into a lever connected to the socket drive. These hammers move the socket drive a little bit at each impact and turn the nut.
The motor housing includes a cylinder, rotor, vanes and end plates. Impact wrenches generate their power by creating a vacuum in the cylinder while forcing air across the vanes to turn the rotor. This generates the torque needed to turn the rotating weight.
There are no seals or gaskets between the cylinder and end plates, therefore the surfaces must be smooth to create a vacuum. The rotor is the only moving part within the assembly and it must be lubricated to ensure it works smoothly.
The trigger controls airflow into the wrench. This is attached to a throttle valve that is steel or plastic and has o-rings connecting the parts together. When the trigger is depressed, air flows from the air line into the throttle valve. It is then passed on to the motor housing that generates the vacuum and turns the rotor that turns the rotating weight that slams repeatedly into the lever connected to the socket drive. The nuts are then cranked down or spun off, and your job is made easy!
With this understanding of how impact wrenches work, you can see the need to keep air dry so that rust and pitting is avoided. The surfaces of the motor must be smooth and clean to create the vacuum needed. The tool must be lubricated as well. When operated without oil, rust will cause the rotor to hang, which will cause the tool to lose power and eventually stop working.
I suppose you're now wondering, ``How does my clicker-type torque wrench know the amount of torque that is on a nut?''
Once again, the principle of how these tools work is fairly simple: A spring in the handle of the torque wrench is loaded against a small cam that is attached to a ``pivot block'' which extends to the ratchet end of the torque wrench. The ``pivot block'' cams over when enough torque has been generated to overcome the spring compression level to which you set the wrench.
The click that is heard is produced as the cam hits against the side of the wrench. The wrench only goes through a maximum of about a 2-degree arc when this spring compression has been overcome. When this happens the wrench becomes a positive tool again, so over-torquing with this type of wrench is common.
The proper way to use a clicker torque wrench is to apply constant force on the wrench and release it as soon as the wrench is actuated. Don't bounce on the end of the wrench when torquing nuts either, because a torque wrench turns into a breaker bar as soon as the pivot block cams over. This is the cause of a lot of fasteners being over-torqued.
These wrenches should not be used as hammers, pry bars or wheel dollies, but they can withstand an occasional drop. Store them in metal boxes to prevent dirt, water and moisture from collecting on moving parts. Because they use a spring, tension should be released before storing the tool for extended periods of time.
I know there are a few ``muscle men'' out there who just pick up the side of the truck when they have to change a tire. But for the rest of us a jack is an essential piece of equipment-it's truly amazing that such a small and relatively light piece of equipment can lift thousands of pounds.
``How does it do that?'' you're wondering. Well, again, the answer is fairly simple.
There are four major components in a jack: the plunger, cylinder, reservoir and base. The plunger is chrome plated, fits inside the cylinder, and is the part that moves up and down. It has a bearing, U-cup and retaining ring on the bottom that create an oil tight seal that prevents oil from leaking along the sides of the cylinder. The cylinder fits inside the reservoir and sits on the base, which supplies a solid surface.
Hydraulic jack oil is housed in the reservoir. To raise the jack, the oil is pumped at high pressure using either compressed air (air-hydraulic jacks) or air pumped by hand (bottle jacks) and forced into the cylinder, which raises the plunger. The base has the release valve that ultimately controls the jack and locks it into place or allows the jack to descend when you're done.
As you can see-like the pressure gauge-the jack is a relatively simple piece of equipment. And like the pressure gauge, it is important to keep the plunger free of scratches, dents, gouges and corrosion so it slides smoothly in the cylinder. Any damage to the plunger gets transmitted to the cylinder since they are in direct contact with each other.
Once the cylinder is damaged, the bearing, U-cup and retaining ring at the bottom of the plunger can also be damaged as the plunger moves up and down. Then you end up with a leaky jack. One of the quickest and best ways to damage a jack is to fail to center the jack under the axle, which creates a stress point between the plunger and the cylinder. The plunger will scrape the edge of the cylinder when it is let down and major damage will occur to the jack.
Also, driving a vehicle off the jack can cause similar damage as well as bend the adjustable screw on the inside of the plunger. If the base is severely bent, the reservoir won't seal and the jack will leak.
Now...about one of those other important questions: For those of you who are still wondering why clocks run clockwise, the answer goes back to before the advent of clocks, when sundials were first used. In the Northern Hemisphere, the shadows rotated in the direction we now call ``clockwise.'' The clock hands were built to mimic the natural movements of the sun. If clocks had been invented in the Southern Hemisphere, ``clockwise'' would be the opposite direction.
Armed with this information, you'll knock 'em dead at that Christmas party!