transmission design

[Rafael90]

Greetings to all students and engineers of this site

I am a mechanical engineering student at the first sockets with design. I was assigned the task of dimensionalizing the whole transmission group of a classic lawnmower (to understand us like that of the enclosed photo). the transmission is very simple: the belt takes power from the motor shaft (the same connected to the blade) and moves it to a pulley that rotates a couple of conical gears that in turn put a gear in place. but the problem does not concern the structure of the transmission nor its dimensioning. the problem is that I have to design the gear so that it has three gears, that at each gear corresponds a certain constant speed. My problem is the following: when the speed of the lawnmower increases also the power that the transmission group absorbs, but the engine cannot be accelerated or I know a way to get to the belt a greater power depending on what is required. How can I solve this problem (as long as it's a problem and not my horny gaffe)?
I hope I have been clear enough in the explanation

 

[Kawa46]

the power that absorbs is the same, the reducer only modifies the combination moment torque-speed angular. if you are in the first or the shorter march you have more torque but less speed. in third that is the longest you will have a greater angle speed but less available torque. for each gear the product angle speed*torque motion will give the power you have in the input to the reducer that is always the same

 

[Rafael90]

That is the problem. My purpose is to design the reducer so that at each gear the lawnmower goes at constant speed. to go at constant speed the power provided to the transmission group must be equal to the resistant one (considering obviously the transmission performance). the fact is that if you increase the speed of the lawnmower also increase the resistant power but I do not know how to take more power to the belt connected to the motor shaft.

 

[calacc]

Does the lawnmower have a blast engine or is it electric? in the first case you can vary the power provided, in the second no, unless you want to put an inverter. and on a lawnmower I do not see it very well...

 

[Rafael90]

It's a blast engine. like the various power delivered without an accelerator?

 

[calacc]

My purpose is to design the reducer so that at each gear the lawnmower goes at constant speed.

perhaps here you meant that engine Tosaerba vada a number of turns constant? ! ?

I don't know, there are things that are unclear to me, can you post the whole exercise?

 

[Rafael90]

No no I meant constant speed. the project is to dimension the transmission so that when the first report is inserted the lawnmower goes to 1,5 km/h, when the second is inserted at 3.5 km/h, when the third is inserted at 5 km/h. practically the purpose was to dimensional a kind of change with a very rudimentary clutch. but what is creating me more problems is to understand how to increase and decrease the power that the belt takes from the engine shaft, since I cannot increase the power of the engine itself

 

[calacc]

but what is creating me more problems is to understand how to increase and decrease the power that the belt takes from the engine shaft, since I cannot increase the power of the engine itself

then, the engine is a blast engine. then it will have its characteristic curve, power-number of turns. Can't you, for example, start by setting the power value to the system, then from the chart you get the value of the number of turns corresponding to that power, and put it at the input of the transmission?

 

[ramjet]

the project is to dimension the transmission so that when the first report is inserted the lawnmower goes to 1,5 km/h, when the second is inserted at 3.5 km/h, when the third is inserted at 5 km/h.

There are a number of errors.
Meanwhile, the speed does not mix it in km/h but in turns per minute.
If you have only the peripheral speed instead of the angle, you should give us the disk diamerto with the blade (which I imagine is 600 mm).

said this size a change with three pairs of dense wheels that have the same interasses (which is the real gearbox) and only at the end you put on the shaft led a pulley that transmits the bike to the conical couple.

your mechanical system is made up of 3 easy sizing subsystems, so.
Good job

 

[Rafael90]

the transmission of the lawnmower serves to make it move alone, not to turn the blade. for that I mix the speed in kilometers times, because I have to move the lawnmower to that speed, not the blade

 

[calacc]

the transmission of the lawnmower serves to make it move alone, not to turn the blade.

And I suspected him... It's a tractor then, not a lawnmower!!! ahahahahah:biggrin::biggrin::biggrin:
Well, I'm struggling to believe that the engine power can't be adjusted. The accelerator's gonna kill us! if you have to go uphill for example how do you do it?

 

[Rafael90]

No no is a moisturizing mower, I have an equal home like that of the photo... I thought the same thing, that is, I couldn't figure out how it could bear the load changes. then I thought of a possible solution, that is also to constant power, if the load due to the transmission the motor decreases turns but consequently increases the torque... if it decreases the power transmitted by the belt (whose pulleys wheel drive solidarity with the motor shaft) increases... Is that possible?

 

[calacc]

if it decreases the power transmitted by the belt (whose pulleys wheel drive solidarity with the motor shaft) increases... Is that possible?

for me here is always the couple to increase, not the power!

the power could increase depending on the load only if there was an electronic system (type cruise control to keep the final speed constant and increase or decrease the load will change the power provided by the engine. But I don't think that's the case.

If you have one at home, you could do a test. You make him work on a corrugated fund and see how he behaves... reasonably according to me uphill you will feel the number of turns of the engine that drops while downhill vice versa. the same speech will be for speed, in each march will charge light variations depending on the load.

 

[Rafael90]

I imagined it was so, thank you very much for confirming my suspicions

 

[calacc]

At the end it is a moving grass, so if you want to work at constant speed the load changes you pay by hand! I mean, you push him uphill and let him go downhill.
are those doubts that often resolve going to the field to cut grass! eh oh to become good engineers is also necessary practical experience.
in good luck for your studies, a greeting

 

[meccanicamg]

View attachment 34371Greetings to all students and engineers of this site

I am a mechanical engineering student at the first sockets with design. I was assigned the task of dimensionalizing the whole transmission group of a classic lawnmower (to understand us like that of the enclosed photo). the transmission is very simple: the belt takes power from the motor shaft (the same connected to the blade) and moves it to a pulley that rotates a couple of conical gears that in turn put a gear in place. but the problem does not concern the structure of the transmission nor its dimensioning. the problem is that I have to design the gear so that it has three gears, that at each gear corresponds a certain constant speed. My problem is the following: when the speed of the lawnmower increases also the power that the transmission group absorbs, but the engine cannot be accelerated or I know a way to get to the belt a greater power depending on what is required. How can I solve this problem (as long as it's a problem and not my horny gaffe)?
I hope I have been clear enough in the explanation

1) the photo attached by you is that of a lawnmower with electric motor with manual push, where the motor axis is vertical and is mounted directly through pin and screw fixing the blade to the shaft (and does not center anything with what you say)

2) if you want to make the change at 3 speeds it is that in the short gear you have more torque but the power is equal since the engine also to burst keeps the accelerator constant and fixed. in the longest march you have that torque is less because you use the power to go in speed

3) If you want to maintain the constant number of lama turns you need to connect the blade cutting motion pulley directly to the lawnmower change input shaft (possibly reduced or multiplied if necessary) and will remain at constant speed regardless of whether you use 1 - 2 - 3 gear to advance. the absorbed power for the cut remains constant and I think it must remain constant because it is not that if you travel slow as advance you have to choose the blade, otherwise you do not take anything but squash the grass on the ground.

4) I think it is necessary to understand how the transmission works if you want to copy it from an existing one otherwise it comes out something meaningless and that it does not center anything with reality

5) according to me the possible transmission is as follows:
- shaft output shaft constant rotation n, constant w, constant c
- pulleys on the engine output that starts at the reducer input
- second pulley connected to the first with trapezoidal belt or synchronous + conical gears or multiplying/reduction system turns for rotation blade
- 3-speed mechanical reducer with fixed interassess + oziosa wheel for rear wheel connected by belt or direct (depending on motor shaft orientation) to wheel axle

 

[Rafael90]

1) the photo attached by you is that of a lawnmower with electric motor with manual push, where the motor axis is vertical and is mounted directly through pin and screw fixing the blade to the shaft (and does not center anything with what you say)

2) if you want to make the change at 3 speeds it is that in the short gear you have more torque but the power is equal since the engine also to burst keeps the accelerator constant and fixed. in the longest march you have that torque is less because you use the power to go in speed

3) If you want to maintain the constant number of lama turns you need to connect the blade cutting motion pulley directly to the lawnmower change input shaft (possibly reduced or multiplied if necessary) and will remain at constant speed regardless of whether you use 1 - 2 - 3 gear to advance. the absorbed power for the cut remains constant and I think it must remain constant because it is not that if you travel slow as advance you have to choose the blade, otherwise you do not take anything but squash the grass on the ground.

4) I think it is necessary to understand how the transmission works if you want to copy it from an existing one otherwise it comes out something meaningless and that it does not center anything with reality

5) according to me the possible transmission is as follows:
- shaft output shaft constant rotation n, constant w, constant c
- pulleys on the engine output that starts at the reducer input
- second pulley connected to the first with trapezoidal belt or synchronous + conical gears or multiplying/reduction system turns for rotation blade
- 3-speed mechanical reducer with fixed interassess + oziosa wheel for rear wheel connected by belt or direct (depending on motor shaft orientation) to wheel axle

1) the image I put was useful just to make it clear that I did not speak of a tractor, but of a moving lawnmower, which therefore does not have a accelerator.

2) I know how a change works, the problem I cannot solve is another. depending on the march, the lawnmower goes at a certain constant speed. to go at constant speed, the power to the wheel must be equal to that dissipated. if however increases the speed, it also increases the dissipated power. My problem is that, since there is no accelerator, I do not know how to increase or decrease the power that the belt takes from the engine shaft

3) I do not want to keep the number of lamas constant by force, even because it rotates to 2800 turns per minute, so if even it should be lowered to 2600 I do not think that would affect its ability to cut

4) the transmission is already copied from an existing one, in fact it is made exactly as you described it in point 5, the only my doubt remains what I have already exposed to paragraph 2

 

[meccanicamg]

If I figured out how it works, you wouldn't have any doubts.

- the change works so that the number of lamas and absorbed power turns are direct and constant
- the tractor should be dimensioned at maximum power, after which you will have a different torque according to the reduction ratio since the power remains constant
- everything must be dimensioned empty and reality will go to fill small diversity
- to increase the speed you need to change gear and not use the accelerator
.... rethink the energy balance of powers that we are not