Re: I don't know
What is there to look for in a used pivot. Does the pipe actually rust or wear through, or is it all just drive train parts. Any one out in the neb. Area that is trustworthy enough to buy from sight unseen.
Re: I don't know
Nebr made a point about the long overhang stress. We have wind.
Look at the welded points above the tower legs and where the truss rod supports are tied to the main pipe. Look for cracks.
look down the inside of the pipe where the rubber boots tie towers together to look for rusting in the bottom of the pipe.
As the others have said the primary issue is fatigue. The stress of bouncing in the wind or swaying of the overhang. Or the pull of temperature changes. The extreme cold temps in the winter can contract a system several inches in a few hours and cause a lot of stress on those outside tower legs upper weld points.
Try to get an idea how much fertilizer has been pumped through it over the years. If you suspect a lot-------inspect the inside of that pipe even if you have to cut a boot off of it.
A lot of life on a sprinkler depends on the quality of the water it delivers. The Nebr guys can correct this but in some areas the systems in some of the river bottoms handle more corrosive water. If that is the case, life is short unless the pipe is plastic lined.
Good clean water and 25-30 years is not unheard of.
Re: I don't know
Just about any reputable dealer takes trade ins. Our local guy has a couple sitting on the lot.
About the only thing I would suggest, is get a machine that is still made. While you can usually retro-fit parts on an old machine, one with a current parts availabilty, that will just bolt on, will save you money and downtime in the long run.
Be sure that when you price, you get a quote that is delivered at the very least, and set-up would be nice, too. You can hire local crews to put up the structure, but whoever wires it , needs to know what they are doing.
Things to look for are any new driveline components, and contactor boxes (also called alignment boxes) on electric drive pivots. The gearboxes and motors are self-explanitory, but the contactor boxes are what turn each individual tower on and off, to keep the pivot properly aligned. If those are going bad, you will have a headache from day one, as an out of line pivot will keep shutting iteslf off.
Another thing to look for, would be fuses vs. breakers. Some of the older machines use fuses to protect the motors on each tower, the newer ones are self-resetting breakers. A lightning storm used to take out a couple fuses on our old pivot, until they were upgraded to self-resetting breakers. My opinion would be, if it still had fuses, they would be old enough they should be replaced anyway. There are electrical contacts in each box, that wear out over time, as the motors cycle on and off.
For a hydraulic pivot, with planetary final drive, you want them to put on new flex hoses at each joint, and guarantee no leaks.
IMO, the most reliable, and easiest to fix pivot out there, would be a hydraulic drive pivot, with planetary final drives. They just work, and keep going. The drawback is that the speed control is at the end tower, instead of the center point (for how fast you want the pivot to make a circle, which dictates the water applied) One advantage to that, is that there is NO copper wiring running the length of the pivot, for anyone to steal.
Those can also be upgraded, to a speed control, that is controlled by a box at the center point, to control it. It does have a small wire running to the end tower, but it is MUCH smaller than on a conventional pivot, much cheaper to buy, not as valuable to thieves, it is only about the size of a standard outdoor extension cord.
The drawback of the hydraulic drive, would be that they are slower to get out of the way of the planter or combine, unless you get the high-speed option, which wasn't avaialable until fairly recently.
One thing to beware of, is that there are people who chemigate, and they are trading in their pivots for new ones, now that they are making a good profit, so you do want to check out the condition of the structure. I should not have to explain what running liquid fertilizer through a pivot can do. Some operators are very careful, to always run the last day or two of irrigation with just water, to rinse everything off, but I have seen others, who just shut the machine off, and leave the fertilizer residue on it all winter.
I personally don't like long overhangs, but some people don't mind them.
The reason I don't like them, is that it gives a big wind something to 'grab' on to, which makes the pivot easier to tip over. Even if it doesn't tip, the constant 'sway' can cause metal fatigue where they mount. A nieghbor's pivot just a mile from my house had the overhang collapse ($12,000 in damage) when one of the support cables came loose.
Re: what is the life span of a center pivot irrigation unit
Most of my pivots were bought well used. Biggest worry for me is the ones made before they galvinized the pipe. (early 1970's) I have some of those spans in a 15 tower machine. In talking to a dealer, biggest problem with rusted pipe is leaks before it gets structly unsound. I am going to change those out for some newer used spans. I plan on using most of those old towers in a small pivot however so they will not get parked.
My pivots start in april and have very llittle down time untill late aug. I apply most of my nitrogen through the pivots. Have not saw any ill effects from that, but may be different if some one ran fertilizer then shut down without running fresh water for awhile.
In my area of nw nebraska, a galvinized pipe will last almost forever in a pivot. I did have a pivot dealer (Kacal in oneil ne) tell me that east of them is a area that has water that the outside of the pipe can look fine and it is rusted out on the inside. I have never saw that. I would think that would not be a problem with buying from a dealer, but may want do directly ask them.
On a big iron auction or something like that it may be good to get some one to look at the machine. At least ask for some close up pictures of the inside as well as the out side of the pipe.
It is common when putting up a old machine to replace the drive train on the last 2 or 3 towers (most milage). If you are paying a dealer to fix tower box problems, often people have them replace the micro switch and the relay just to make sure . The extra cost will not pay much milage and labor cost. I still have some of the origninal relays working in towers that were made in the early 70's (Zimmatic). fix any drive line joints, worn ends of drive shafts may need to have the drive shaft replaced or sawed in 1/2 and the worn ends welded togather so you have good square ends on the shaft. wheel gearboxes with loose input shafts need either replaced or new bearings and seals put in. I wouldn't expect to need to replace the tower electrical boxes on a valley or a zim, some of the others may be a differnet story. One thing to be aware of on a old zim. they changed the design of the cam that runs the microswtches. if that was changed out to the newer style, the wires may need to be reversed on the run microswich. Some will actually run without moving the wires, however the safety microswitch may be almost ready to trip all the time. I have known some zim tower boxes to get replaced because the guy working on them did not understand what was going on. (if the tower is in alingment and the safety microswitch is not centered in the run slot, that is the problem). My limited knowledge of the lockwood systems is it would be worth it to get a tower box conversion and replace all the tower box systems. Use Zimmatic, Valley or Nebraska irrigation conversions. On a old zimmatic that is disassembled, it is a good idea to replace the little plastic arm under the tower box. (that hooks to the linkage that runs the microswitch).
My experience with hydrlic pivots, as well as watching what happened with the new ones the neighbors put in, I would personaly not even consider one of those. Yes they do have problems and from listening to the neighbors, they are not always that easy to fix. When running 24/7 a leak that uses 5 gallon of oil a day is not easy to find. years ago, I moved a used olsen pivot and had lots of problems with leaks in the oil line joints. Would expect that would not be so big of deal with a brand new machine. Hydrlics don't leak much when new but look at any piece of equipment that uses lots of hydrlics and has lots of hours on it and I would be amazed if you don't see some places where there has been oil leaking.
There are set up crews that travel, that dealers hire to help out in busy times. a dealer should be able to find one of those crews for you. It is not rocket science to set one up, but if you have never been around running pivots and never been around one being assembled, it may be a learning experience. Using new gaskets is a must do.
Long overhangs as others have mentioned work, however they commonly have problems, better to add a short span and shorten up the over hang to 40 ft or even 20 ft. I have a pivot that at one time had a 80 ft overhang. It has crashed five times. I now have shortened it up to 45 ft and put in a longer span down the pivot aways. I have not had any other spans tip over or fall down from sand in the pipe or from wind damage.
It may be differnet in differnet soils, but I would stay away from wide tires. Go with the tall tires if you expect to have problems with deep wheel tracks, getting stuck, low spots where water may commonly stand