I made a winch by using an hd hex motor with Ultraplanetey gearbox the ratios are two 5;1 and one 3;1. Then I put 30 tooth plastic gear with the motor and a 45 tooth plastic gear with the spool. Then I had to pass the UHMWPE cord from two motor mounts so I can balance the weight but could be they are slowing the motor down .The only problem I got is that it’s very slow in getting up .Is it ok to lift directly from the winch as I’m afraid I make damages .
1 Like
Can you add a couple of pieces of information?
-winch drum diameter
-expected/designed lifting speed
-expected/designed lifting weight
One of those things where without knowing the goal and a critical spec makes helping hard.
For the record, I suggest using pulleys to redirect your line, instead of motor mounts. But let’s look at the overall system first.
2 Likes
There would be a lot of friction passing the cord through those motor mounts like that - we’ll get to that later. With two 5:1 and one 3:1 stage, followed by a 30:45 stage, you have a 112.5:1 overall reduction on a motor with a free speed of 6000 RPM and a stall torque of .105 N*m.
The free speed tells you the maximum speed the motor can run when there’s no load on it. 6000 RPM /112.5 = 53.3RPM. That is already fairly slow - if you have a 1" diameter spool on your winch, then that means it pulls in about 2.8" per second. To raise something 1 foot would take over 4 seconds, at the motor free speed. Since you’re actually lifting something, your motor will be running slower than free speed, so I would guess closer to 6 seconds to lift 1 foot. Note if your spool is smaller, then it would be even slower, while a larger spool would go faster!
That brings us to the output torque. For a mechanism like this, I would look at what I need to lift and work backwards - so long as you have enough torque, it’ll lift, so you can then adjust your gear ratio knowing that limit. If the total amount I need to lift (game piece + mechanism) was 10 lbs, then I can use that to figure my ideal gear ratio. I need to know what I’m using for a spool, to start. Lets use that same 1" spool from the paragraph above. That means my output stall torque on the motor needs to be at least 1" * 10 lbs = 10 inlbs. For convenience, we’ll convert the motor’s .105 Nm to .92989 inlbs, or about 1 inlbs. That would mean I need a total gear ratio of somewhere around 10:1 in order to barely be able to lift it. I like to build in a safety factor, meaning I have at least 50% more torque than I need, so I would aim for something like 15:1 or 20:1 in this case - both of which would go significantly faster than the 112.5:1 you have currently! Of course, the weight you’re lifting (for example, mechanism + game piece) really matters when figuring this out!
Now, if we build it that way and it doesn’t work, we need to figure out why. Either something was off in our calculations, our weight measurements were wrong, or we didn’t take something into account - in this case, it would likely be friction from those motor mounts - you’d need to increase the gear ratio in order to overcome both the weight and the friction in the system.
Now, with the background out of the way, to get to your question:
That depends. Many teams have lifted 150 lb FRC robots straight from the winch without problems. But to do that, you need to have the winch properly supported. AndyMark’s Climber in a Box Winch Kits can provide some great inspiration. With these kits, the winch shaft is fully supported on both sides, with the whole thing having a small width. It avoids having a cantilevered shaft, which can easily bend, or a very long shaft, which could also bend even if supported on both sides. Support your winch shaft the same way, and it’ll likely be ok. By supporting, I mean having some bearings on either side so your motor doesn’t take any side load.
So, take a look at how the winch is mounted, strengthen that if needed. Remove the extra friction and consider if you can decrease the gear ratio, and you can get that speed up!
9 Likes
I’m trying to get our team to realize that to move forward you have to work backwards.
4 Likes
Thanks all for the replies.
The winch drum diameter is around 3/4 of an inch.
I would like to lift the robot which it weights approximately 22 pounds and would like to do it in 10 second in a height of 171 cm. The only gear ratios I got are from Rev robotics which are 5:1,4:1 and the 3:1.
The other problem I faced when I did 5:1, 4:1 and the 3:1 gear ratios the motor starts going backwards when it reached the desired height . Sorry for this but I’m not that much technical in these things.
Which gear ratio better to change the one in the motor or the 30:45 ?
Given your requirements:
- 22 lbs
- 10 seconds
- 67 inches
And your givens:
- 3/4" winch diameter
- 6000 RPM motor free speed
- ~1 in*lbs motor torque
You have what you need to figure out your desired gear ratio.
Lets start with speed:
- 67 inches in 10 seconds is 6.7 inches per second
- 3/4" diameter drum has a 2.35" circumfrence
- 6.7 / 2.35 = 2.85 revolutions per second
- 2.85 RPS = 171 RPM
- 6000 PRM / 171 RPM = 35:1
- This means you need a 35:1 or less gear ratio to hit your speed target. Thus your current 112.5:1 is about 3x too high to meet your speed target, in a perfect world.
Now, lets look at torque:
- 22lbs * 3/4 in = 16.5 in*lbs
- 16.5 inlbs / 1 inlbs = 16.5:1
- you need a 16.5:1 ratio or larger to meet your torque needs.
So, your sweet spot for an overall gear ratio is somewhere between 16.5:1 and 35:1. Keep in mind that you don’t want to be running the motor too close to stall torque (16.5:1), and you won’t hit your free speed number because you have a load on the motor (35:1).
If it were me, I would aim for a total reduction of 25:1 - 30:1. Try going with a 5:1, 4:1, and the 45:30 - that’ll give you a total of 30:1. If you want to try faster, you could replace the 4:1 with a 3:1, that would give you a 22.5:1 - perhaps a little too close to stall for me, but you won’t know until you test it!
9 Likes
I will try both and see how it goes.Do you think that when I reach the desired height it will keep the robot in place or it can go backwards ?
Most likely you’ll need to add some sort of braking device. Depending on what the exact scenario is, you may be able to use low power to the motor to hold position, or add a ratcheting device.
I’ll also add a plug for my AMB robotics calculator ambcalc.com, which has a mechanism calculator that does a lot of these calculations automatically.
3 Likes
Sorry for my late reply. I did tried both the ratios but didn’t succeed . Now I’m trying to experiment by making a winch by itself and I’m trying two motors spinning the same shaft as I’m attaching the picture below. I put two 4:1 ratios on the motors and 1 45 tooth gear in the middle and a 30 tooth gear on the motors but still I cannot lift the 22lbs.
The winch drum looks much larger than the 3/4 inch diameter you described earlier in the post. This change increases the torque required to lift your 22lbs, which is probably why you can’t lift it now. Try decreasing the diameter of the drum.
2 Likes
To expand on this, consider how the equations used previously change if we change the diameter - lets call it a 3" diameter now, as an example.
So, if that new diameter was 3" (You’ll need to measure this, it’s just a guess!), then the previous ratios would not provide enough torque to lift 22lbs - they would be less than half of what you need.
The solutions to this are:
- change the spool diameter back to your 3/4" from the original description
- Change the gear ratio
Adding a second motor doubles your power, but you still need your gear ratios to be higher - with two motors and the numbers above, you might be able to get by with 35:1 gear ratios on them, but you’re nowhere near that with what you described.
4 Likes
So to understand it’s better to have smaller spool rather than a bigger one. I tried to calculate the ratios on what I did and guess I bring them somewhere near 51:1 , is it better to calculate in inches or mm ? Sorry for asking much as I’m just trying to learn . If I put two 5:1 ratios on both motor and I leave the 30 tooth gear and the 45 tooth gear I end up with a 37:1 ratio if I’m making the sum well. Thanks
Yes as I forgot to mention that the drum I used it’s bigger as I thought it would help in speed,but guess I was mistaken. It have a diameter of 8.04 inch.
What I didn’t understand is how you get the 35:1 ratio as when I’m making the 22lbs*3/4 inch =16.5 .
Not better, just different. If we assume friction doesn’t exist (it does, but that’s fine), then your spool size has the opposite impact as your gear ratio. A larger gear ratio means more torque and less speed, while a larger spool means more speed and less torque. The ratio between torque and speed is basically inversely linear, no matter what you do - to get more of one, you get less of the other.
So while your larger spool did give you more speed, it reduced the torque to the point where it couldn’t lift it.
Keep in mind none of my math used 8” for the spool diameter, so you’ll need to figure that out.
Your basic equations are:
The two red equations could be written as a single equation easily enough, but breaking it out this way helps show what each part of the equation is actually doing. Ditto for the blue!
The motor specs we have are 6000 RPM and 1 in * lbs of torque - those are the starting points for the red and blue equations (for a single motor… if you have two motors, then you double the torque while the speed remains constant)!
Your requirements are 22 lbs, 10 seconds, and 67 inches of travel - those are the lifting force and speed, or the ending points for your equations.
In the middle you have your gear ratio and your spool dimensions. You can’t solve for one without knowing the other. Sometimes, you have a spool in mind, so you plug in the numbers from your spool to figure out your ratio. Other times, you may have limited ratios available, so you plug those in to see what spool sizes you can use.
Try playing with those equations a little, along with your available gear ratios and spools, and see what you can learn!
2 Likes
Try plugging in your current spool diameter of 8 inches to see what you get. Then plug in a spool diameter of 4 inches, then 2 inches, then 1 inch, then 1/2 inch to see what you get in each case and compare the results.
1 Like
Thanks for your quick reply. I’m just started on making this as a hobby and to learn what I didn’t as it’s something out of my knowledge as I left school very young so I’m trying to figure out how things work . So excuse me if I post silly things.
It’s great that you are trying things and asking questions. Keep doing this and you will be learning lots of stuff. Soon, you will be helping answer people’s questions 
1 Like