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I am new to this site and have a problem to solve in an area that I’m pretty lost in. I can usually figure stuff out with the help of the internet and some digging but not getting real traction so hoping some folks smarter than me can help!

I’ve built a sawmill and currently using an ATV winch to raise and lower the 350lb cutting head. It works great and very accurate but drawback include its very slow at around 24 rpm not to mention it must be drawing heavy amps so battery not lasting long.

I’m hoping to get some recommendations on a 12 V dc motor to select for my job. I don’t know what torque specs I need to adhere to let alone battery life and amp draw.

Specifications Include:

  • 350lb weight to move on lineal axis
  • Cutting head is welded to 1” Acme rod nuts
  • Cutting head travels up and down on 2 Acme rods that are linked together with no 40 chain and sprockets.
  • Acme rod is 1” diameter and 4 threads per inch
  • max travel of head is around 30” up or down
  • Typical motor activation might include turning on to raise or lower +/- 15 inches then followed by no activation for 2-3 minutes. Followed by 1-4” incremental changes up or down every 2-3 minutes.
  • Battery should have capacity to run for 8-10 hours in the above mentioned cycling example.

Any recommendations for conserving battery life, torque needed to lift at 60-100 rpm? Force? What is force?

Thanks a ton for your help!!

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Transistor
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SawmillBuild
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  • @NMech thanks for your bullets on my question..,much more clear! Any comments on the solution? – SawmillBuild Jul 01 '21 at 05:00
  • Depending on the friction of the drive nut on your Acme thread, you can get around 700 lbs of axial force with only 10 lb-ft of torque on a 1"-4 Acme. 4 TPI = 0.25" linear travel per rev, if you only want 4" of travel in 3 minutes that's 16 revs/3 min, or 5.3 rpm. 5.3 rpm & 10 lb-ft torque = 0.01 hp. So you don't need too beefy of a motor mechanically, someone knowledgeable in electric motors/batteries should be able to take it from there. Torque needed to raise/lower is independent of speed. Power is the product of those two parameters. – jko Jul 01 '21 at 11:52
  • Hi jko, thank you very much for your comments. This is great information. However, let me clarify something....re: 4" of travel....I'd like 4" of travel in one minute. Raising and lowering the mill cutting head occurs each time a board it cut. So sometimes the saw may be all the way lowered after finishing cutting a full log. When a new log is loaded the saw head needs to be raised to the top of the log ie. if log diameter of 22" the saw head needs to travel 22" continueously until the height is reached. A cut is made and then the head is lowered typically by one inch, cut, one inch, cut etc. – SawmillBuild Jul 01 '21 at 12:51
  • I think I really need rpm's to be around 75. Re: friction...only thing I could find was using steel nut dry=.15-.25 and using same oiled=.11-.17. Does this chage the lb-ft torque needed? Also, is 700lbs axial force the same as lifting 700 lbs of mass on the axle? Thanks so much for your help? – SawmillBuild Jul 01 '21 at 12:54
  • If you want 4"/min travel is going to be 16 rpm, still very doable with a DC motor. 75 rpm fine too. As coefficient of friction goes down, you get more axial force (lifting force up the axle). Running steel nut dry at .25 coef. friction you only need 5 lb-ft of torque to get 390 lbs lifting. – jko Jul 01 '21 at 15:25
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    @jko thank you for this clarification. 5 lb-ft torque falls in an range where I can still purchase an affordable motor. – SawmillBuild Jul 01 '21 at 16:20
  • @jko I found a motor on ebay...It's slightly lower rating than what you are calling for but do believe it should do the job. It's 12v 50 RPM with switch for forward, reverse, and off positions. Rated torque is 6 N-m or 4.4 ft-lb. I estimated that at rated load of 60 watts the motor draws 5 Amps so with my 12v battery with rating of 63 Amps/20 hr the battery would last for 12 hrs 36 minutes. So thanks so much again for all your help! – SawmillBuild Jul 01 '21 at 17:12

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You are very close to an automotive power seat. They come in a variety of configurations . I have not had one out for many decades . I can only suggest you go to an auto salvage yard to see what is available.

blacksmith37
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  • Years ago, GM used a single 12V motor and a transmission with three flexible cables; back & forth, front up & down, rear up & down. The steel cables drove a rack and pinion or an Acme thread rod. The seat plus two people would easily be 350 lb. The transmission would select which of the three cables to turn. – blacksmith37 Jul 01 '21 at 15:06
  • I like it! But I'm too by the book when I have a significant problem to solve. I've already purchased a winch that has served its purpose for a short time. The next purchase I need to be spot on. Thx for the input though! – SawmillBuild Jul 01 '21 at 16:19