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September 2010 Archive for In the Shop

RSS By: Dan Anderson, Farm Journal


In Simple Terms--Potentiometers

Sep 27, 2010

 Potentiometers, sometimes called "pots," are used frequently on modern farm equipment. In simple terms, potentiometers connect to a shaft or linkage that in turn is connected to some part of the equipment that pivots or moves in some way. A soybean platform automatic header height control is a good example. There is a shaft that goes all the way across the skid shoes underneath platforms that rotates as the skid shoes flex. A potentiometer, connected to the end of that sensing shaft, allows the combine to "sense" the position of the shaft/skid shoes and raise or lower the platform as necessary.

Potentiometers merely measure movement and translate the movement into a voltage reading. Of the three wires that go to potentiometers, one is supplied constant voltage, one is ground, and the other is "signal" that varies with the position of the potentiometer's shaft. The amount of signal voltage varies as the potentiometer's input shaft/linkage is rotated.

You often hear mechanics talk about "calibrating" potentiometers. Calibrating is the process of moving the part of the machine the potentiometer is attached to through it's range of motion. This "teaches" the machine the upper and lower limits of voltage the potentiometer will see during the machine's operation. On the bean platform, the potentiometer is calibrated by lowering the platform all the way down, then raising it until the sensor pads no longer contact the ground. The combine "learns" the voltages for upper and lower pad movement, and uses that information to automatically adjust header height.

If something changes about the way the machine operates---the platform hits a rock that bends the sensing shaft, a belt wears into a potentiometer wire, a potentiomenter linkage gets bent--that may change the movement of the potentiometer's shaft and allow it to report higher or lower voltage than normal to the combine. Any variation outside the previously determined voltage ranges can cause the system to throw a warning code and possibly shut down---even as little as 1/1,000 of a volt. 

At that point it's up to the operator or mechanic to determine if there is something wrong that needs repaired (broken wire, broken linkage) or if the system simply needs recalibrated to clear the system and "start over."

My experience is to recalibrate first, maybe several times, tinker with "cheating" the upper and lower setpoints during recalibration, and then replace/repair if needed. Potentiometers are nifty little gadgets, but they can drive you crazy if you let them.

In The Shop: Easy Does It On U-Joints

Sep 18, 2010

 Y'know those little u-bolts that hold the needle bearing caps in the yokes of universal joints? How tight do you suppose the nuts on the ends of those u-bolts should be torqued when installing a u-joint?

I've seen guys use air wrenches to torque them down. I've watched guys stick a big screwdriver through the driveshaft yoke to hold the driveshaft so it couldn't turn while they put all their weight on a 1/2-inch breaker bar to tighten the nuts on those u-bolts. 

Pat Fagen, u-joint and driveshaft guru at The Axle Exchange in Des Moines, jokes that when he installs u-joint u-bolts, he puts axle grease on his hand before he uses that hand to hold the driveshaft while he tightens the clamp bolts. "That way the shaft turns before I can overtighten the nuts on the u-bolts," he says.

Fagen explained that few u-joint u-bolts require more than 20 or 30 lb-ft of torque, and that some smaller u-bolts measure their maximum suggested torque in pound-inches.

"If you overtighten the nuts on those u-bolts you can damage the bearing cap and the needle bearings inside," says Fagen. "A lot of needle bearing failures in u-joint caps can be traced to over-tightened u-bolts. All joking aside--when you're tightening those nuts, hold the driveshaft with your (ungreased) hand and torque them to recommended value or till they're good and "snug." There's no need to crank on them till they're good and "tight." Good and tight is probably over-tightened."

In The Shop: It Worked For Me

Sep 12, 2010

 I can't promise it will work for you, but adding a "ground chain" to a combine reduced how often I had to clean windows on a combine while harvesting soybeans.

In a previous life I operated a combine for 16 harvests. I can't remember where I picked up the idea, but I decided to experiment with "grounding" the combine frame. I bolted a length of light chain to the combine's lower frame so it dragged on the ground between rows. The theory was that all the belts and pulleys and spinning components create static electricity that builds up in the combine's frame, isolated from the ground by the rubber tires and poly skid shoes on the grain harvesting platform. Since static electricity "attracts" dust, the theory was that removing the electrical charge would reduce the build-up on the machine's windows of fine, powdery dust.

The first year I tried my experiment, I was skeptical. I still had to clean the cab windows daily. Then one afternoon I noticed dust was building up on not only the cab windows, but the entire machine. I had to stop twice that afternoon and wipe down the windows to be able to see well. That evening, while fueling and servicing the machine, I noticed that my "ground chain" had come off. I installed another chain and next day went back to cleaning windows only once a day.

Adding a ground chain didn't cure all my dust problems, but for me, I could see a visible reduction in the amount of fine, dry, "clingy" dust that accumulated on not only cab windows, but the entire combine. 

If you try adding a ground chain, be sure to grind off all the paint to bare metal where you fasten the chain to the frame to ensure good contact. And, I never noticed a big difference in dust reduction when harvesting corn, probably because the higher amount of stalk residue between the rows interfered with good chain-to-soil contact.

I know it sounds a little goofy, but...that's my opinion and I'm stickin' to it.

In The Shop: The Difference Between a Good Press and a Bad Press

Sep 09, 2010

 I've used a lot of hydraulic shop presses over the years to press out bearings and dis-assemble/re-assemble machined parts. Some were elaborate, high-capacity 50-ton monsters, some were home-made out of scrap iron and an old hand-pump hydraulic jack.

Aside from sheer capacity and safety, the thing that made various presses "better" had nothing to do with the actual press itself. The presses that were most useful to me had a good assortment of seal drivers, pieces of pipe, chunks of iron and other metal stock stacked nearby that gave me lots of options for doing things safely and quickly with the press.

One reason the press in our shop is so handy and useful is because my co-worker Sparky is a pack rack who can't walk pass a scrap iron pile without making a "withdrawal." He has scrounged, cut, and sized more than 50 pieces of pipe, tubing, angle iron, round bar stock, square tubing, and thick strap iron and arranged them near the press. It is SO fast and handy to have an appropriate piece of metal within arm's reach of the press. Safer, too, because when we have the right diameter and length of metal stock for a press job, we're less apt to precariously stack smaller, shorter pieces to get the job done.

So, what I'm saying is, when I visit a farmer's shop and need to borrow his press to do a repair, I don't really care if it's a fancy 50-ton with a crank-adjusted table and foot-powered hydraulic system, or if it's a little home-welded H-frame press sitting on a work bench with a 3-ton bottle jack for a power source. What impresses me is if there's a supply of metal stock handy to make that press quick, safe and extremely useful.

in The Shop: Bearing Busting

Sep 06, 2010

 There are many ways to remove worn or damaged bearings from shafts and housings. I'm not bragging or confessing, but here are some methods that have worked for me:

-Hammer and chisel/punch. If you can get behind a bearing and use a hammer and punch to drive the old bearing off the shaft, you're having a good day. No muss, no fuss, and as long as you don't mar the shaft, installing a new bearing is a breeze. Use safety glasses--bearing races are hardened metal that often shatter or chip when struck.

-Air hammer and pry bar. This is the technique I usually try first because if it works, it's fast and clean. Run an air hammer against the end of the shaft while prying on the bearing or its housing. Make sure you've cleaned all paint and corrosion off the end of the shaft, and the buzzing action of the air hammer will often slide the bearing smoothly off the shaft. Use safety glasses and wear gloves to prevent injuries from shattered bearing housings.

-Cut-off wheel/die grinder. If the bearing is exposed you can use a die grinder with either a cut-off wheel or a cutting tip to surgically cut through the outer and inner races. It takes time, but is a very "clean" way to remove bearings. Again, use safety glasses.

-Gear pullers and pry bars. If you have room to get the jaws of a gear puller behind a bearing or its housing, a gear puller is another clean alternative that's fairly quick. Bearing splitters and other special tools are nice if you can get them into place and IF you have the right tool for the job.

--Acetylene torch. Okay, long-time readers knew I would get to this one eventually. Burning off bearings with a torch is crude, dirty and potentially dangerous. But it is the fastest way to remove a bearing I've found. I always have a fire extinguisher on hand and liberally dampen with water the area adjacent to the bearing I'm about to destroy. There will be flames, sometimes a LOT of flames, when bearing grease ignites. There will be lots of slag and sparks and small pieces falling and flying around. There is the potential to damage shafts, housings and wiring harnesses in the vicinity of the bearing being removed. It takes practice to remove bearings without nicking the shaft or bearing housing. But it can be done, and it's fast and functional when done with care and caution. When possible, I try to let shafts and housings cool before I install new bearings, otherwise the hot components can melt the "factory grease" out of the new bearing.

There are dozens of other ways to remove bearings. The "right way" is the way that allows you to get the job done comfortably, safely and as fast as possible. For me, I always try to buzz a bearing off with an air hammer and pry bar, then... get out the garden hose and keep it handy because there are going to be lots of sparks and flames.

In The Shop: Reprising Praise For Farmers

Sep 01, 2010

 I'm repeating the same basic theme from a post I made last winter, but it's worth repeating: The skills and knowledge you have as a farmer are more than you comprehend.

If I say, "Turning on the endrows causes combines to throw over because the machine is temporarily running empty," you understand what I'm saying. Today it took me 10 minutes to explain what I meant to a non-farmer. Before he entirely understood what I was trying to say I had to backtrack several times and explain to him what endrows were; why combines ran "empty" on endrows; how combines actually work; why they don't work efficiently if less than full of crop, and...oh yeah, I also had to explain what I meant by "thrown over" when I talked about grain getting "thrown over."

I've had the same problem in the spring when talking about plants per acre, seed depth, the relationship between seed spacing in the row and seed population per acre. Talk to a non-farmer about sprayers and spraying (gallons per acre versus gallons per minute, and the relationship between flow rate and ground speed) and their eyes glaze over in seconds. 

You could do the same to me if you started talking about how the plumbing is buried around your farmstead, or the way the electrical system is wired between various buildings. You either grew up with it or installed it or oversaw the installation. Heck, if you had to write an owner's manual for your farm it would take multiple volumes.  There'd probably be a separate volume just to explain how to bypass the mercury switch to get the grain dryer to work until an electrician can actually get there to fix it.

Think of it this way: If you had to cram all your farming and business knowledge into the head of an 18-year-old town kid, it would take far, far, FAR more than the four years it takes to become an engineer. Or the 8 years it takes to become a lawyer. (FIll in your favorite lawyer joke here...)

So this is another salute to the "owner's manual" to your farm that you have stored in your head. Never underestimate the skills and knowledge you have. As a farmer once ruefully told me, "If I had to pay myself for what I know, I couldn't afford myself." 

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