Posted by: akueddy ® 03/13/2007, 19:39:28 Author Profile eMail author Edit

Hi guys, I am trying to do an analysis of an electric carjack, i know the load, i know the torque require at the end of the jack(ive calculated it to be 5Nm). i know the typical electric car jack uses 12V 10A pmdc motor (about 3500rpm no load speed and stall torque 0.4Nm). I know that to maximize the output torque of the motor i need a gearbox that'll give out the optimum mechanical advantage. I dont have any clue how the design of an eletric car jack gearbox which is on the market, but i know the dimensions of it,15x2x4cm, below is the link of my design base on the dimensions. units are in cm and all the gears are fix to the shaft/motor. https://img483.imageshack.us/img483/9297/gears23ax2.jpg Anyway, the gear arrangements will increase the motor torque 25times(assume super efficient).and so this will give out 25*0.4=10Nm. Well its larger than the required torque,so it'll work but wait..thats assuming if the gears are 100% efficient, and at stall torque.. So my question is can anybody like suggest any other gear arrangements which gives out more than mine or speculate what are the gear arrangements on the electric car jack on the market right now? thank you

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Re: guessing gear arrangements?
Re: guessing gear arrangements? -- akueddy	Post Reply	Top of thread	Forum

Posted by: zekeman ® 03/14/2007, 11:29:54 Author Profile eMail author Edit

First off, from your data, if you need 5Nm at the jack, and you are using the described motor, I get a torque constant of .033Nm/amp, which means at 10 amps the motor will put out .33Nm and at that point on the motor curve you need a gear ratio of 5/.33= 15. Since the speed constant of the motor is .033V/rad/sec, ( same number as torque constant). At stall, the current is .4/.033=12 amps. Now at 10 amps the motor speed will only be 2/12 of 3500 or abot 600 RPM. And if the gear ratio is 15 then the jack RPM will be 600/15= 40 RPM.

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Re: guessing gear arrangements?
Re: guessing gear arrangements? -- akueddy	Post Reply	Top of thread	Forum

Posted by: randykimball ® 03/13/2007, 21:03:39 Author Profile eMail author Edit

First, I'd use a worm gear or a screw jack. If you use a stack of gears and you loose power you stand a good chance of the jack dropping. I had a trailer once with an electric tongue jack. It was a screw jack using a gear reduction to crank the screw. When the power was off the screw would not turn due to the friction in the gear box trying to spin the motor at high speed. Figure out at what RPM the motor is designed to operate at. Figure out at what RPM you want to spin the screw to lift the load. Use the thread pitch per inch of the screw per distance in what time. The slower the screw turns per the motor RPM the less load on the motor, but the longer it draws power. As an inverse the quicker the screw turns per motor RPM the more current you draw for less time.. so the slower screw RPM wins to a point at which it is still realistic time wise. Then this tells you the ratio... example: 25 screw RPM per 1200 motor RPM is.. 25/1200=48 You would want to set up the gears to run at 48 to 1. To get a gear reduction you must put a larger and a smaller gear on the SAME shaft, driving the larger and letting the smaller drive the next stage. You multiply the ratio of each large gear to each small gear on the same shaft (stage) by the ratio of the next stage, by the next stage... this gives the final gear ratio. ... or you could buy a reduction gear head from a supply house and couple it to a selected motor intended to couple to it for an easier build. Then couple the gear box to the screw thread.... or... you could purchase a electric trailer tongue jack and adapt it. These save you from having to build a gear cage, have shafts made and keyed to match the keys in the gears, having to purchase several matched gears, selecting the gear style - pitch - width - material - lubrication method - and much more. does any of this help??? The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Tue, Mar 13, 2007, 21:14:57

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Re: Re: guessing gear arrangements?
Re: Re: guessing gear arrangements? -- randykimball	Post Reply	Top of thread	Forum

Posted by: akueddy ® 03/13/2007, 23:27:49 Author Profile eMail author Edit

hi, I am aware that if i know the load,i know the torque required,i need to proceed with the motor specs and gearbox design. I think i may have not specified what i really want, sorry about that. The main thing i am working on is to analyse the current electric car jack which i have no access to(no money). So i just speculate the design of the gearbox, and try to figure out what the gear configuration is on it. I have done some research on the net of a typical 12V 10A motor, and the specs are about the same, 0.4Nm stall torque, 3000-4000rpm. The lifting time of current jack on the market is below 60 seconds. I try to use my gearbox design and my calculations are just at the margin, and so i just hope that someone can point out the real gearbox design inside the electric car jack thats possible produce more torque, doesnt really matter to me about the speed of lifting,as long as the torque output is larger than the torque requirements.

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