Got my stubborn blade bolt off... Now what?

Discussion in 'Homeowner Assistance Forum' started by leejp, May 22, 2007.

  1. leejp

    leejp LawnSite Member
    Messages: 129

    I've got a set of Meg-Mo's for my 36" walk behind. Hope these blades live up to the billing. I'm cutting once every 5 days and barely keeping up with the rapid grass growth.

    This was the first blade change on the new mower, The bolts were stubborn. My impact wrench (cheap coleman) didn't budge them so I soaked in PB blaster overnight and used a 4' piece of pipe to get them off.

    Now what? Once the Meg-Mos are installed, how should I tighten? I'm going to use some sort of anti-seize compound for sure. But which one?

    Also... should I hand tighten or use my impact wrench which couldn't get the nut off in the first place. Some cursory searches here suggest that either would do but this is the first time with the Meg-Mos and I'm a bit nervous about slipping.
  2. SSS 18734

    SSS 18734 LawnSite Member
    Messages: 182

    I tighten my blade bolts to around 80 foot-pounds of torque using a torque wrench. I can still easily take them off by hand, but I don't have to worry about the blades flying off either!
  3. bohiaa

    bohiaa LawnSite Fanatic
    Messages: 5,220

    GREAT advice..... also I allways put wheel bering grease on the threads of the bolts............ ALWAYS...........

    MOW PRO LAWN SERVICE LawnSite Bronze Member
    Messages: 1,568

    Versachem or permatex both are top.
  5. MarcSmith

    MarcSmith LawnSite Fanatic
    Messages: 7,157

    if you apply grease or any other type of lubricant to your threads, then your torque values mean nothing.

    I never applied grease or oil to any blade bolts....but when you are changing/sharpening the blades out a couple times a week its not an issue...
  6. leejp

    leejp LawnSite Member
    Messages: 129


    Torque is the same regardless of the lubricity. Do you mean the likelihood that it'll back out?
  7. MarcSmith

    MarcSmith LawnSite Fanatic
    Messages: 7,157

    On the contrary....

    From a strength and preload standpoint the ideal steel fastener would have a plain black finish, (sometimes called a light oil finish). This finish produces a fairly consistent K-value and does not compromise the strength of the fastener. This finish would be unacceptable on a bike since it corrodes easily. The common solution is to apply a zinc or cadmium plating to prevent corrosion, and apply a conversion coating such as chromate to keep the finish looking nice. If a more decorative finish is desired, the fastener is usually polished and chrome plated. Plating causes problems with high-alloy steels due to hydrogen embrittlement, if appropriate measures are not taken after plating to "bake out" the hydrogen. This is especially true of chrome plating which tends to lock in the hydrogen. Plating does not adversely effect the mild steel used for 8.8 fasteners. The torque-tension relationship is greatly affected by plating due to its effect on the friction coefficient. Cadmium plating reduces the friction by 25% and zinc plating increases the friction up to 40%. This requires a corresponding 25% reduction or 40% increase in required torque for the same tension. Stainless steel fasteners have a friction coefficient about two times the corresponding plain steel fastener. This does not mean that stainless fasteners require double the specified torque since they usually cannot achieve the strength of a steel fastener.

    Thread lubrication is another variable that affects the torque-tension relationship. I performed experiments on approximately 20 lubricants and found that the lubricant can change the torque to achieve a given tension by a factor of two (up or down!) I found that super clean fasteners or those lubricated with light lubricants like WD-40tm require a high torque to achieve the desired tension. Fasteners lubricated with oil such as motor oil and the oil found on black fasteners require a medium torque. Fasteners lubricated with extreme pressure grease or anti-seize paste require the least torque. Interestingly, I found that Loc-tite has about the same lubrication action as light oil. I discussed this with the manufacturer and they said this was by design so that the torque-tension relationship would be approximately the same as plain steel fasteners with normal manufacturing oil.

    Remember the bolt is essentially a spring. over stress it and it won't spring back... strip the threads and you'll need a torch...
  8. leejp

    leejp LawnSite Member
    Messages: 129

    OK I see...

    Since I'll remove the blades 1~2x/year I think I will put some permatax on the threads and tighten using the lowest setting on my impact wrench . Quite a few folks here have used this method with no problems.
  9. Let-it-mow!

    Let-it-mow! LawnSite Member
    Messages: 91

    I always thought you were supposed to add "running torque" to the specified torque value given for a fastener. Running torque is the torque it takes to turn the bolt before it bottoms out.

    so if the spec is 50 ft-lbs, and a close-fitting grade 1 fastener takes 10 ft-lbs to just start it turning, you add the 10 and 50 to get a spec of 60 ft.-lbs.

    If you lube a bolt and have almost no running torque, then you just stick with the spec'd value.
  10. MarcSmith

    MarcSmith LawnSite Fanatic
    Messages: 7,157

    with the blades and other stuff this probably isn't much of an issue (lubing the bolts) But when it come to critical stuff, like car wheel, engine blocks, brakes never see them lubing bolts for the exact reason of over stress/under stress and not getting the required clamping force.

    Ideally all nuts and bolts should spin and hand tight(no tools) usually results in 10-20 inch lbs of torque which is pretty insignificant in larger applications like mowers and such.

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