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Thread: Designing Test Joint - 500 ft lbs

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    Question Designing Test Joint - 500 ft lbs

    Hi all,

    Problem
    I'm working on designing a test joint to test torque output of various power tools: impact wrenches, direct drive tools, click wrenches, etc. The joint must be able to withstand thousands of rundowns at max torque of 500 foot pounds.

    Design



    The TJ will be designed similar to my drawing, a bolt that threads into an outer sleeve component, with washers to soften the joint (I.E. more cushion).

    I'm thinking an ASTM A193B8 Bolt will work. 1 inch diameter, 3 inches long

    My main question is this...
    How do I determine the type and quantity of washers to be used? The operator will be able to remove washers as necessary in order to harden the joint, but I want to make sure that I can get it soft enough.

    Thanks,
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    Last edited by Braren; 01-14-2015 at 02:19 PM.

  2. #2
    Administrator Kelly Bramble's Avatar
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    How do I determine the type and quantity of washers to be used?
    I believe that beveled washers are typically used. The number used is dependent on the spring rate of each beveled washer.

    Are you going to use a bolt fastener force sensor?

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    Beveled washers I agree would work best, but oddly enough, the company that makes the Tester/transducer sells test joints that have flat washers. These are very hard test joints which makes the torque readings of pulse tools and direct drive power tools very inconsistent, especially with higher torque applications. Another supplier does use beveled washers in their design, however.

    I actually won't be using a bolt fastener force sensor for this. The test joint is just going to sit in a torque tester that has a 1" female square and built in torque transducer.

    This is what I'm kind of thinking.
    Torque: 500 ft lbs
    Bolt Maj. Dia: 1"
    Approx Clamp Load: 30,000 lbs

    Belleville Washer Flatload: 860 - 42,088 lbs

    Designing for the max clampload, I would want to have enough washers such that the total Flatload >= Max Clampload, right? What do you think would be best, a few washers with large flatloads or many washers with smaller flatloads?

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    Or could I benefit from a combination of washer flatloads?

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by Braren View Post
    Or could I benefit from a combination of washer flatloads?
    Yes you could though the change in measured or applied torque would be much more per unit of turn to the fastener than with bevel spring washers.

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    Im admit that I have essentially no experience with the max torque testing of power tools so please be patient with me because I am having a bit of difficulty understanding the function of the bolt and flat washers.

    From my view point, the power tool will best exhibit its max torque capability once the bolt is fully seated.

    As a result, I would think that you would fully seat the bolt prior to starting any max torque measurement test. Alternatively, based upon your drawing it seem to me that making an adapter consisting of a bar with the appropriate hex size machined on one end and the square adapter machined on the other end would work as well except that the hex end of the unit would not be replaceable when damaged.

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    If this test fixture is going to be used repeatedly then you should think about a Grade 8 bolt rather than using any Stainless alloy. Stainless Steel has a tendency to micro-weld and gall and will essentially weld itself together after only a few turns unless you use an anti-seize compound, however the anti-seize compound will affect your torque readings!

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    Quote Originally Posted by Macgyver View Post
    If this test fixture is going to be used repeatedly then you should think about a Grade 8 bolt rather than using any Stainless alloy. Stainless Steel has a tendency to micro-weld and gall and will essentially weld itself together after only a few turns unless you use an anti-seize compound, however the anti-seize compound will affect your torque readings!
    it is a grade 8 bolt; ASTM A193B8. B8 = grade 8.

    john

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    Quote Originally Posted by JAlberts View Post
    Im admit that I have essentially no experience with the max torque testing of power tools so please be patient with me because I am having a bit of difficulty understanding the function of the bolt and flat washers.

    From my view point, the power tool will best exhibit its max torque capability once the bolt is fully seated.

    As a result, I would think that you would fully seat the bolt prior to starting any max torque measurement test. Alternatively, based upon your drawing it seem to me that making an adapter consisting of a bar with the appropriate hex size machined on one end and the square adapter machined on the other end would work as well except that the hex end of the unit would not be replaceable when damaged.
    We're actually not really going to be testing the max torque of the power tools (although sometimes that will be the case). The bolt & washer assembly will fit into a torque tester, that records the torque values that it sees. It has different modes of operation: peak torque, which returns the max torque it sees (typically for direct drive power tools); a torque tracking mode, which displays the torque being applied in real time; and a pulse tool mode which has multiple filter frequency settings. The pulse tool mode is the one I care about. If you don't know, a pulse tool is basically another word for impact wrench, think of what pit crews use to change tires. The tool applies very rapid pulses of applied torque until the tool clutches out at a certain torque setting. These tools are very common in manufacturing because they are fast, low cost, and require very little exertion by the operator. However, because the tool "pulses" and applies torques very rapidly, they aren't the easiest tool to set to a particular torque (there is no standard), and they aren't as accurate as direct drive, or DC tools. For something like a click wrench your method would work just fine, but pulse tools are another beast.

    Pulse Tools work pretty well for relatively soft joints, but for hard joints (like the design with flat washers above) are very inconsistent. The tool slams down into the joint so quickly, that the torque readings are inconsistent. What I am trying to do is buffer the rundown so that it requires more turns of the bolt to reach final torque.

    With the current test joint, the tools torque out in less than 2 or 3 pulses, but you want to be somewhere between 7 and 15 pulses.

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    Quote Originally Posted by Macgyver View Post
    If this test fixture is going to be used repeatedly then you should think about a Grade 8 bolt rather than using any Stainless alloy. Stainless Steel has a tendency to micro-weld and gall and will essentially weld itself together after only a few turns unless you use an anti-seize compound, however the anti-seize compound will affect your torque readings!
    Right, I will definitely use a grade 8 bolt.
    I will actually probably intentionally grease the crap out of the bolt and test joint. The trickiest part I think will be finding the right relationship between Torque and clamp load, because as you mentioned, it depends on many factors.

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    Braren,

    Now that I understand the issue at hand it appears that belleville washers are your best bet for their compact height and high load capability. Here are a couple of comments and an alerts about your selection and application using them.

    First, with regard to your specific choice of the number of washers, I expect you already are aware of the two configurations in which they are applied, in that nesting two washers face to face doubles their assembled load capacity and nesting two washers edge to edge doubles their assembled compression travel. So, by combining those two arrangements in a stacking series you can achieve both the maximum load capacity => the 30,000 lb test load and achieve the number of bolt rotations you want before the washer set flattens.

    The alert I want to mention is that when stacking more than two bellevilles edge to edge the guidance of the washers becomes an issue if you are placing them along the threaded section of your test bolt. If the washers are not externally guided and centered by by an external smooth bore sleeve, the inner edges of the washers can catch on the thread O.D. edges; and, result in erratic loading during tightening and also damage the test assembly bolt threads.

    In your case, if you are designing your own fixture, there are two alternatives:
    The first is to place a washer guiding c'bore with an I.D. slightly larger than the maximum flattened O.D. of your belleville washers and slightly deeper than the maximum height of your washer height in the top of your fixture block with the tapped section for the bolt engagement below that c'bore.

    The second is to select a test bolt with an upper shank section longer than the total unloaded height of your washer assembly. Next, drill a hole (slightly larger than the bolt shank diameter) in the top face of your assembly block to a depth equal => (the bolt shank length minus the flat height of your washer assembly) and then tap the bolt thread below that hole.

    I hope this information will be of some help.

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    Braren,

    I am adding this post after my above one because I don't know how familiar with the application of belleville washers and I want to miss giving you the best information I can right up front to at least try and prevent one of those "I wish you had told me that before" instances. In this case it is additional general information about how to best install a belleville stack.

    First, in general, it is best to design the stack such that the outer edge of the bottom belleville is face down on the lower mounting face and when doing this it is also a good idea to have a hardened and polished flat washer between the belleville and the lower mounting face to prevent wearing of the contacting face when the outer rim of the belleville moves in and out under load changes. Assuming that the inserted flat washer is fully supported its thickness is not important.

    Second in most cases with a bolting application where the bolt is rarely removed and reinstalled it is acceptable to allow the upper belleville inner edge to contact the base of the bolt head. But you obviously have a special case with your repeated testing cycles where the small inner washer edge can cut and gall the bolt head face at it contact line. My suggestion for you is, if at all possible, to arrange your stack so that the upper washer outer edge is facing upward and install then install another hardened flat washer between the bolt head and this upper belleville. Of course, as opposed to the lower flat washer this top one will have to be designed to transfer the full 30,000 lbs load from the belleville's O.D. to the bolt head face diameter.
    One more note regarding your question in an earlier note about the preferred number of bellevilles to use. In order to reduce the amount friction between the washer stack's contact with either the bolt or a surrounding O.D. washer guide to reduce the number belleville edge to edge misalignment the fewer the number of bellevilles used the better.

    One last comment regarding my earlier post recommended giuding arrangements. Since sometimes matching a standard manufactured bolt's overall, shank and thread lengths to a design can be a challenge you should feel free to add to either end of your washer stack whatever number of flat washers that are required to assist matching your bolt and your fixture design.

    Finally, at this point, I have exhausted all of the design tips that I recall from my experience with both applying standard bellevilles and designing custom ones for special applications. If you have any questions just post them and I'll answer them as best I can.

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