Any mechanical device is gonna need maintenance from time to time, and drilling rigs are no different. One good thing about them: They generally will outlast almost every other mechanical object you own, with a little care. The basic designs have been around long enough, and the bugs worked out to the point where I can't think of much that lasts longer, with the possible exception of alimony payments.
One thing I see a lot of drillers do is confuse maintenance with repairs. Let's get this straight: Repairs are what you do when you don't do maintenance. It's kinda like when my bride and personal terrorist, bin-Lottie, forgets to put new batteries in the smoke detector. She doesn't know when supper is done, and I have to fix the fire damage in the kitchen. A little maintenance would have solved a bigger problem.
On rigs that I appraise, I generally see a couple of recurring problems, both related to poor - or nonexistent - maintenance. One is the mud pump. It doesn't take much sand in the gear end of a mud pump to end up costing thousands of dollars. Sand is like liquid emery cloth and eats brass the way termites feast on a hickory stump. Leaking packing glands shoot high-solids mud straight into the gear end. A small leak is best fixed right away, with maybe a look at the oil in the gear end. A big leak that ran all day means you need to drain AND flush the gear end, and refill with clean oil. Don't forget to use the type of oil that is recommended by the pump manufacturer.
Example: A lot of pumps recommend using 85w90 oil, but drillers think that going to 140-weight oil will help when the pump is getting some age on it. This is a bad idea. 140-weight oil has a different viscosity than 85w90.
Since most pumps are splash-lubricated, the 140-weight oil won't splash as much and won't lubricate the pump properly. Not only that, the higher viscosity will suspend abrasive solids longer, allowing them to work their way into moving parts and do their damage.
Another commonly seen problem is rotary table pinion bearing problems. The problem develops like this: First, the U-joint retainer nut loosens up. Next, the pinion gear pre-tension in the rig gear moves, destroying the oil seal. Then all the oil runs on the ground. This usually doesn't take long, and the driller may or may not notice it. Since the rotary is out of oil, it won't leak any more. End of problem, right? Wrong!
A leaking rotary oil seal is a $5 part and a big hassle to put in, but a new ring and pinion, and bearings can make a significant impact on your bottom line. Another thing that causes significant wear on rotary pinion bearings is wear in the telescoping drive shaft on rigs with retractable tables. When these develop enough “slop,” they side-load the pinion bearings beyond their design and severely shorten their life. Most original factory designs had the telescoping male end of the drive shaft toward the rotary, and the receiver, or female end, toward the rotary transmission. I think this is bass-ackwards. With the male extension end toward the rotary, it is a magnet for abrasive mud. Simply reversing the shaft, end for end, shields this area. Another advantage of having the extension tube receiver at the forward end of the shaft: You can put in a drive-shaft “carrier” bearing to take a lot of the slop out of the assembly. Too bad the rig manufacturers didn't ask me about this years ago - I've prolonged the life of many-a-rig with this trick.
Another pretty neat trick, if you have good, tight seals on the rotary, is to run premium synthetic oil. I have a friend that has run a rotary table continuously with synthetic oil for more than 30 years without a breakdown or a rebuild! The length of service and savings on rebuilds more than paid for the oil.
Oh well, it's Sunday afternoon, there's fixin' to be a race on TV, and I feel the call of a recliner. To sum up: Grease costs much less than bearings! With a little maintenance, we all ought to be able to keep our rigs running 'til we're on Social Security. Wait! Maybe I'm wrong. Looking at the way the “guvmint” is spending money like a drunken sailor, it'll be broke before most of us get a penny!