worm shaft thrust force calculation
worm shaft thrust force calculation
(OP)
Aloha,
I have a 270in-lb moment acting on a worm gear and I'm trying to calculate the thrust force extered on the shaft of the worm. The worm gear is about 3.3 inches in diameter.
For a static thrust load calculation the thrust force exerted on the worm shaft is about 270in-lbs/1.67in = 160lbs.
I was told that I should perform a dynamic thrust load calculation since the thrust force will increase while the worm is rotating. The worm will be spinning at about 24RPM max. The equation I was given to use is this one.
T=P/tan of lead angle
where T=thrust force,P=input torque/pitch radius of worm
Well, I know my output torque must be at least 270in-lbs. My worm gear ratio is 40:1. So I first divide 270in-lbs by 40. I get 6.75 in-lbs. I then multiply this by 1/worm gear efficiency. The efficiency I calculated for this worm gear set is 15%. By multiplying 6.75 in-lbs by 1/.15
I get 45 in-lbs as my input torque on the worm.
Then to solve for P, I divide 45in-lbs by 0.5in, the pitch radius of the worm. This gives me 90in-lbs.
Then to solve for the thrust force I divide 90 in-lbs by tan(4.08deg).
This gives me a thrust load of about 1260lbs. This dynamic thrust load does not seem to be right since by static calculation is only 160lbs.
Can anyone point out what I am doing wrong with this calculation? Is there another way I can calculate the thrust load exerted on the worm shaft? I'll be using this thrust load calculation to choose the proper thrust bearings for my worm gear box.
Mahalo
I have a 270in-lb moment acting on a worm gear and I'm trying to calculate the thrust force extered on the shaft of the worm. The worm gear is about 3.3 inches in diameter.
For a static thrust load calculation the thrust force exerted on the worm shaft is about 270in-lbs/1.67in = 160lbs.
I was told that I should perform a dynamic thrust load calculation since the thrust force will increase while the worm is rotating. The worm will be spinning at about 24RPM max. The equation I was given to use is this one.
T=P/tan of lead angle
where T=thrust force,P=input torque/pitch radius of worm
Well, I know my output torque must be at least 270in-lbs. My worm gear ratio is 40:1. So I first divide 270in-lbs by 40. I get 6.75 in-lbs. I then multiply this by 1/worm gear efficiency. The efficiency I calculated for this worm gear set is 15%. By multiplying 6.75 in-lbs by 1/.15
I get 45 in-lbs as my input torque on the worm.
Then to solve for P, I divide 45in-lbs by 0.5in, the pitch radius of the worm. This gives me 90in-lbs.
Then to solve for the thrust force I divide 90 in-lbs by tan(4.08deg).
This gives me a thrust load of about 1260lbs. This dynamic thrust load does not seem to be right since by static calculation is only 160lbs.
Can anyone point out what I am doing wrong with this calculation? Is there another way I can calculate the thrust load exerted on the worm shaft? I'll be using this thrust load calculation to choose the proper thrust bearings for my worm gear box.
Mahalo





RE: worm shaft thrust force calculation
T=P x tan of lead angle
where T=thrust force,P=Input torque/pitch radius of worm
This gives the component of the torque producing force acting along the shaft direction (axial thrust).
RE: worm shaft thrust force calculation
Although, your reply makes perfect sense, When I calculate P x tan of lead angle I get 6.41 lbs. I know that this isn't correct since my static calculation is 160lbs. Would it be 160lbs plus 6.41 lbs?
RE: worm shaft thrust force calculation
Looking at the "Mechanical Engineers Data Book" by J.Carvill
I quote the following formula's:-
Tangential force on worm (wFt) = axial force on gear (gFa)
these forces are = 2*Tw/(Dw)
where Tw = worm torque
Dw = worm pitch circle diameter
further:-
Tangential force on gear (gFt) = axial force on worm
(gFt) = |(cos B - u*tan X)|
|---------------- | *(wFt)
|(cos B + u*tan X)|
where cos B = cos of normal pressure angle
tan X = tan of the worm helix angle
u = coeff of friction.
However looking at the formula you are using and I quote:-
"Then to solve for P, I divide 45in-lbs by 0.5in, the pitch radius of the worm. This gives me 90in-lbs"
If you divide 45in-lbs/ 0.5in then you get 90lbs
not 90in-lbs so I think there is something missing or incorrect with the formula you have been given.
Regards desertfox
RE: worm shaft thrust force calculation
Would that equation be in a mechanical engineers handbook as well?
Ohh desertfox, there was a typo.
What I said was "Then to solve for P, I divide 45in-lbs by 0.5in, the pitch radius of the worm. This gives me 90in-lbs"
What I meant to say was "Then to solve for P, I divide 45in-lbs by 0.5in, the pitch radius of the worm. This gives me 90lbs".
So the units come out correct.
mahalo nui loa!
RE: worm shaft thrust force calculation
I have never seen the formula that you are quoting however
I found this formula for Dynamic loading in a book called
"Machine Design theory and practice"(ISBN 0-02-329000-5)
Fd = ((1200+Vpg)/1200)* Ft
where Vpg = pitch line speed of gear in feet/minute
Ft = transmitted load as determined by the input
horsepower applied to the gear.
best regards
desertfox
RE: worm shaft thrust force calculation
RE: worm shaft thrust force calculation
The correct values will satisfy
gear ratio * tan(lead angle) * worm pitch radius = worm gear pitch diameter / 2
2. The formula trust force = input torque / worm pitch radius / tan(lead angle) is only valid where efficiency is 100%. Otherwise, the formula is trust force = efficiency * input torque / worm pitch radius / tan(lead angle).