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Discussion Starter #1
I need to change the blades on my rotary cutter and the spec. says torque to 450 ft. lbs. These wrenches cost $350. Do you guys use a torque wrench or do you use an impact driver or what? Any info would be appreciated.
 

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Are you sure you have the right spec? Thats alot of torque for the blades, what size (bolt size not wrench size) are the bolts or nuts that your trying to torque. Also whats the make/model or at least size of your rotary cutter. I've had trouble in the past with manuals not converting metric to ASE properly.

Brad
 

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Discussion Starter #3
The manual says that the bolts are 1 1/8 - 12. and to torque the blade bolt lock nut 450 ft-lbs. My cutter is a Land Pride RCR1548. The manuals are also available online. Any help would be appreciated. It gives these torque specs in the blade changing section as well as in the torque values in the back of the manual.
 

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Wow I guess it is that much, you could try an impact but I'm not a fan of guessing when it comes to large peices of sharp metal spining at high speeds. See if you can borrow a wrench from someone. Or put 225lb guy at the end of a 2ft breaker bar lol. If you had a GOOD impact with proper air pressure you might be able to get away with it. Just remember what your working with.

Brad
 

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Discussion Starter #5
Well thanks for looking. I'm gonna see if I can rent one in town and go from there. I really don't want to get hit in the head with a flying blade because I decided to guess if it was tight enough.
 

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drill a 3/4" hole in one end of a 4'-2" long piece of 2" square tube and tighten a 3/4" bolt through the hole. Slide your 3/4" Johnson bar and socket into the other end and use your 1/2" drive torque wrench and 1 1/8" socket on the bolt. Set the torque wrench to 112 foot pounds. That should give you 448+/- 15 ft. lb.

Ideally you want exactly 48" center-to-center between the two sockets for accurate torque.
 

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Thats a good idea tudor I had forgotten about that trick, only thing I never liked about doing it was the amount of variables that can come into play. But since this is gonna be a regular tool for service I guess you could weld a cheap 1/2" socket into the one end and 3/4" short extension onto the other so your not fighting 3peices.

Brad
 

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You're right TooManyGT. I just threw out the quick and dirty, close enough for ____ method. Refinement is up to the individual.

I used to tighten coupling bolts to 650 ft. lb. with a 4' long 3/4" drive torque wrench. That was 35 years and 50 lb. after I quit doing chin ups. It's easier to pull up than pull down. I'm all for better leverage.
 

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Oh I understand, I've done some pretty ugly down and dirty/rigged stuff when I'm in a pinch. I just remember trying to hlod three things that could move while tigtening them down lol, it wasn't pretty. I just wanted to throw some ideas out since it sounds like he'll be doing this somewhat often. Leverage is our friend when working on equipment!

Brad
 

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I used a 1" impact gun and a impact socket that fit. One hint that I always use on those bolts because of all the dirt and corrosion that happens is to use never-seize or similar product on the bolts and nuts. They can be real bears to get off the next time that you want to replace or sharpen the blades.
 

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...is to use never-seize or similar product on the bolts and nuts. ....
I agree but you have to watch the torque wether it's spec'd dry or lubed. You tighten up a dry spec'd bolt that you've lubed and the actual torque goes way up....Mike
 

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I agree with a lot of what's been said before, and I'll throw another couple of ideas into the mix. You might want to start watching for a used torque multiplier, as these work great for higher torque applications, and make a good investment for large fastener work.

I have to spec bolt tightening in a lot of things I design, and have read a lot on the topic. Tightening bolts with an impact gun is one of the most inaccurate methods available. It really gives you no way to assess how much you have stretched the bolt, which is what you are doing when you tighten a fastener. Impact guns are very unpredictable as to how much torque you are really applying and will vary a lot from day to day. Tightening by torque is also a fairly inaccurate method because of variation in the friction and lubrication, but it is easy to specify and measure, so commonly used. On a lot of structural joints, you will often use a method called "Turn of the Nut" where the fastener is turned a specific amount after the joint is snugged. This can give very high preloads, and the fasteners are often only usable for a few cycles of tightening and loosening before fasteners are damaged and must be replaced.

If you don't want to invest any extra money, I'd probably contact the manufacturer and see if they can give you a turn of the nut (or bolt) tightening method. You might have to push their support people a bit, but their engineering people can probably give you an answer if you can get the question to them. If you can borrow a torque wrench of enough capacity (maybe rent one from a local auto parts store for a few hours), you can generate your own information. Make sure that the surfaces and fasteners to be tightened are clean. Snug the bolt and mark the orientation of the head. Use the torque wrench to tighten the fastener, and measure how far you have turned the head. Do this several times on each spindle, and you can see how far and how consistent the angular turn range is. Be sure you lubricate the fastener each time you tighten things when developing your turn range. You can then tighten things by measuring the angle of turn with any wrench.

Best of luck! If I can turn up any more information, I'll post it.
 

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I dug around a bit more and found this technical reference on bolt tightening, and am including a link below. Around page 7, they discuss different tightening methods. Uncalibrated impact wrenches can vary by + or - 60% of the torque being aimed at. This is a lot worse than most people would guess, and can be enough to damage fasteners, or leave a joint loose. Bolted joints, especially mechanical joints on equipment, are often designed with a lot of excess clamping capacity, and so are tolerant of mis-torquing. Engine and gasketed joints have less margin and are less tolerant. Understanding fasteners and tightening can give you a lot of understanding about how much care you need to pay with any given joint design. I wholeheartedly understand and agree with taking as much care as you can in following the specs in assembling and maintaining machinery, but sometimes a bit of knowledge can save you from having to buy a one use tool.

http://www.skf.com/files/880426.pdf

One other note, if you develop a tightening angle as I described above, please keep in mind that it only applies to the joint you measured it on. The angle value is affected by fastener diameter, length, and the joint structure, so you really need to develop this number individually for each joint you are working on.
 

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Discussion Starter #15
Thanks for all the info loremaster. I think I will try renting a torque wrench capable of 450 ft-lbs and count the turns. My question is, every time you replace the blades you are supposed to use a new bolt and lock nut. Will this effect the amount of turns since it will be a new bolt and nut every time? for example differences in the hardness of the teflon in the nut?
 

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Unfortunately, that kind of depends on the material the bolt is made of relative to the torque. It's always safest to replace fasteners, and you should always do that if the instruction manual tells you to, as that is typically a sign that the fastener is being pushed near its limit. If nothing about that is mentioned, it's a judgement call. If your bolt is grade 5 or A325, the standard chart torque for these is 975 lbs lubricated. Since you are not approaching those values, you could re-use the fastener, as long as the threads look good and there is not evidence of damage to the fastener. If the bolt is a grade 2, then you are torquing it to the max, and you probably should replace it. There is nothing wrong with torquing a fastener to less than the maximum allowable for its grade. Here's a chart for bolt and nut markings

Lock nuts are trickier. If the nut seems to spin on without much resistance, replace it. Typically you can get a few cycles out of a nut without significant degredation of the locking material. In fact, if I remember right, locknuts are often tested for torque retention after five tightening cycles. From your description, the fastener is a nylock, and so is probably good for a few cycles. These do shift the torque somewhat, but you can actually measure the free running torque (the torque it takes to turn the fastener before it bottoms against the joint). This is typically a small percentage of the torque that the fastener is being tightened to, so the change in torque value (or tightening angle) is small and can be ignored. As the free running torque decreases, the amount of preload being applied to the bolt increases.

Since the preload is basically proportional to the amount of rotation when tightening the fastener, you can actually test how much it would change the angle. If you have a small torque wrench and take your time, or better yet a dial type torque wrench, you can measure the free running torque. Once the fastener is snugged up, see how much angle you have to turn it to double that torque value. You will see that this is a very small value.

If you EVER see damage to the fastener, especially stretching in the threads (usually at the thread to shank transition), give the fastener a flotation test. That is, throw it in the nearest deep body of water. If it floats, it might be worth retrieving as a novelty. This test is only to be performed with solid steel objects.

If you are ever in doubt, replace the fastener. It's just not worth the chance of damaging the machine or injuring someone if a fastener fails. If the fasteners are expensive, you can also do some checks by measuring the length of the fastener (and recording it) before installation. You need to be at three decimal place on this measurement, and be careful of how the calipers are registering on the ends of the fasteners. If you re-measure it after use and find it has stretched, you should replace the fastener.

Here's a chart for bolt and nut markings.
http://www.americanfastener.com/technical/grade_markings_steel.asp

If you have any questions about this or want any better explanation, let me know and I'll do what I can. I like teaching people to really understand what goes in to some of the design of their machinery.
 

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Discussion Starter #17
Wow, I really appreciate all of the info you provided. I learned a ton. If I have any more questions I will hit you up. I actually found a Diesel shop in town that has a 1" torque wrench capable of like 1000 ft-lbs. They hardly ever use it and the owner of the shop said I can borrow it whenever I want. So I think thats what I will do just so I have the piece of mind that i am not going to kill myself or someone else with a flying blade. :thThumbsU
 

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That's great news. I think after a few cycles, you'll find you are comfortable with the joint and how far / tight you have to make things. It's typically only 1/4 to 1/2 turn from snug to torqued, and now you'll think about it and remember this every time you work on that joint.
 

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That's quite an education, Loremaster. Seems my dad knew what he was talking about all those years ago. Just old farmboy common knowledge.

But, ktfbgb, in your shoes, especially given the task at hand, I'd definitely use that torque wrench.
 

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Excellent tutorial!! :thThumbsU:thThumbsU

:thanku:
 
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