Acme leadscrew vs timing belt getting really high numbers

In summary, the conversation discusses different methods of linear positioning for a CNC project. One method involves a stepper motor and leadscrew, while the other uses a timing belt arrangement. The conversation also addresses the difference in mechanical advantage between these two methods and how energy can be used to calculate the axial force produced. The efficiency of the leadscrew is also mentioned.
  • #1
Taiden
9
0
Hey all,

I'm on summer break so my engineering brain is almost completely turned off. So I would not be surprised if I am making some kind of silly error. I'm trying to compare different methods of linear positioning for a CNC project.

------------------------- Part 1

OK, so here's the scenario.

We have a stepper motor that is able to produce 30 in-lbs of torque.
We have a leadscrew whos manufacturers states that 0.031 in-lbs of torque will "lift 1 lb"

(Not used in calculations, but it is a 3/8-12 acme two start with 43% efficiency)

If I do:

30 in-lbs * ( 1 lb / 0.031 in-lb) = 967.7 lbs axial force

This seems astronomically high.------------------------ Part 2

We have a timing belt arrangement producing linear motion by being constrained (from translation) by a drive pulley on the same stepper motor, a free spinning pulley bringing the belt into proper tension, and a cart attached to one "side" of the belt.

The motor produces 30 in-lbs of torque.
The drive pulley has a diameter of 0.900".
The system is 94% efficient.

30 in-lbs * (1/0.450 inch) * 94% = 62.67 lbs axial force

This seems reasonable to me. The answer from Part 1 absolutely does not.------------------------ Part 3

Leadscrew has 0.166 inches of linear movement per rotation
Timing belt assembly has 2.827 inches of linear movement per rotation

The ratio between the two (TB:LS) is 16.87:1 (ignoring efficiency so far)

The ratio between Part 1 and Part 2 (TB:LS) is 15.44:1 (not ignoring efficiency)How is it possible that the linear displacement to angular displacement ratio is vastly different than the axial force to torque ratio? The leadscrew is magically exceeding it's mechanical advantage?
 
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  • #2
I was ignoring that frictional forces at the screw will increase as the axial load increases.

Anyone know how I can take this into consideration when I go to calculate the axial force produced at the nut with 30 in-lbs of torque at the screw?

Here's the data sheet: nookindustries dot com/pdf/NookInchAcmeScrew.pdf

Page 4 is what you want. On my pdf viewer it's listed as page 21, not sure why.

3/8-12 two start rod with a plastic nut is what we're going for

Thanks all
Luke

PS: sorry for the 10 post link workaround but I figure that's for spam and I need to link to the datasheet for this question
 
  • #3
Taiden said:
How is it possible that the linear displacement to angular displacement ratio is vastly different than the axial force to torque ratio? The leadscrew is magically exceeding it's mechanical advantage?
Putting torque on a threaded screw provides a huge mechanical advantage. Needless to say, there's a conservation of energy, so although there is a very large mechanical advantage for the lead screw, you have to turn the screw a very large number of times. The screw has to be rotated 12 times to move it just 1 inch.

Take your torque of 30 inch pounds and determine the amount of energy it puts out after rotating 12 times. In other words, your motor is producing 30 inch pounds of torque which is equivalent to a force of 30 pounds, 1 inch from the center line of the shaft. So the total distance the 30 pound force rotates through in one revolution is 30 pounds times the circumference (2" * pi). That gives you 188.5 inch pounds of energy. Multiply by 12 and you have 2262 inch pounds of energy produced. If we use that energy to rotate the screw 12 times, we will have produced some force over a distance of 1 inch. Per your calculation, the screw has produced 967.7 inch pounds of energy, the rest we can assume has been given up as heat due to inefficiency. Now if we divide 967.7 inch pounds by 2262 inch pounds, we get an efficiency of only 43% which is exactly the number given by the power screw manufacturer. Seems to work out! :smile:
 
  • #4
THANK you for that post. Our engineering professors don't teach us how to solve problems, they teach us how to follow book steps. I never once considered using energy to find the solution but it's obvious now how easy and effective it is. I'm going to look more into this tomorrow afternoon when my brain is awake so I can do it out and follow it 100%

Thank you again
Luke
 

What is the difference between an Acme leadscrew and a timing belt in terms of getting high numbers?

An Acme leadscrew and a timing belt are both mechanisms used for linear motion in machines. However, the main difference between the two is in the way they convert rotary motion into linear motion. An Acme leadscrew uses the helical threads on its shaft to move a nut along its length, while a timing belt uses teeth on its surface to engage with pulleys and move the belt in a linear direction.

Why do some machines use an Acme leadscrew instead of a timing belt for high numbers?

One reason is that an Acme leadscrew has a higher load capacity compared to a timing belt. This means that it can handle heavier loads and still maintain its accuracy and precision. Additionally, an Acme leadscrew can also achieve higher speeds and acceleration, making it suitable for applications where high numbers are required.

Can a timing belt be a better choice than an Acme leadscrew for high numbers?

In some cases, yes. A timing belt is better suited for applications that require high speed and rapid acceleration. It also has lower inertia compared to an Acme leadscrew, making it more efficient in these situations. However, a timing belt may not be as accurate or precise as an Acme leadscrew, so it may not be suitable for all high number applications.

Are there any downsides to using an Acme leadscrew for high numbers?

One potential downside is that an Acme leadscrew can experience backlash, which is the play or movement between the threads of the screw and the nut. This can affect the accuracy and precision of the movement, especially at high speeds. To minimize backlash, precision machining and proper lubrication are necessary.

Which one should I choose for my machine - an Acme leadscrew or a timing belt - if I need high numbers?

The choice ultimately depends on the specific requirements of your application. If you need high load capacity and precision, an Acme leadscrew may be the better option. On the other hand, if speed and low inertia are more important, a timing belt may be a better choice. It is recommended to consult with a mechanical engineer or conduct thorough research to determine the best option for your specific needs.

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