Tension, springs, force, gears 3

[confused290181]

Hi Guys,

My company use an arrangement where two probe parallel arms are connected with two tension springs with a measured force at measure head set to 4.5Kg. Spring/probes are open and closed by a smc cylinder driving a cam between the probe arms.

The new design removed the cam now the cylinder drives one probe away from the other and the probes are geared together so they move apart.

However this gearing is causing the force required to get 4.5 kg at the head to be increased ie using a scale weight on one arm force is set to 10Kg but on a load-cell at the measure head only shows 4.5kg

I know this is a little sketchy im happy to answer any questions you guys have for a new guy

Cheers

 

[MikeHalloran]

So... the new design is working, but you don't understand why?

Perhaps you could post an assembly drawing or a photo?



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi confused290181

I'm confused to, please provide some drawings or sketches of the set up.

regards

desertfox

 

[TheTick]

If the question is not important enough to merit a picture, it's probably not important enough to merit an answer.

 

[confused290181]

Thanks for the response guys ,

I will post a picture when im back in on Monday im sure it will make sense then for you experts.

to the tick I was hoping that perhaps the problem was so obvious that it did not need a picture thus not giving away any company information on the web.

 

[handleman]

One thing that is confusing to us is that you haven't asked a question yet. You have only described your situation and then offered to answer questions.

-handleman, CSWP (The new, easy test)

 

[desertfox]

handleman

I think the OP wishes to understand why the force as increased, at least thats how I see it,but I could be wrong,
perhaps when he posts the pic the OP can confirm his question

desertfox

 

[confused290181]

Sorry guys I will post pics on Monday then all will become clear,

Anyway old cam design using a scale to set the weight to 4.5 kg when I put a load-cell between the probe it also shows 4.5kg
so far so good

On the new design where the probes are geared together I have to set the springs 10.2Kg on the scale to give a load-cell value between the probes of 4.5Kg

Scale measurement is basically pulling on one of the probe arms ( as close to measure point as poss) until probe separates by 1mm ( using a digital fisher-man scale)

the loadcell measurement, I put a sub minature loadcell between the probes.

Note 1, I have only had the load-cell a couple of days to help with this problem.

The probes measure very thin film weights and the amount of force is critical, I think that the new design could be seriously flawed.

Note 2, Even though by setting the probes to an increased force seems to work as-far as the correct pressure at he loadcell is concerned the machine is still not performing ie RR and correlations etc so I would really like to understand this

Thanks for your patience

 

[desertfox]

Hi confused290181

Firstly are both springs set individuallly to 10.2kg or each set to 5.1kg, secondly are they tension springs because if they are tension springs, then they would be trying to rotate the gears and seperate the probe arms, so you would not have to pull on them with a scale to seperate them in my opinion unless I am misunderstanding your sketch.
If the springs were compression springs then I see that the probe arms would close up and clamp onto your load cell.
Now if you were to pull a probe arm away, then that would compress the springs and increase the spring load and you would obtain a reading on your scale.
Now the reading on your scale would depend on how far away from the gearing centre you are, ie the further away from the gear centre the lower the reading on the scale, the near to the gear centre you pull with the scale the higher the reading, its just basically levers.
What we need is the spring stiffness and the pcd's of the gears, also the overall dimensions of the probe arms and relevant centre distances from load cell to gear centre and springs to gear centre.
I think your problem is just one of levers.

desertfox

 

[MintJulep]

Let's start with kg is not a unit of weight.

I really can't understand or even begin to imagine what your device looks like from your descriptions.

Regardless, it sounds like the cam design and the gear design have different mechanical advantages.

 

[handleman]

Do you know what FBD stands for?

-handleman, CSWP (The new, easy test)

 

[confused290181]

Thanks deseryfox

You are correct about the compression springs.
And the rest of your message agrees with measurements Ive taken,

However Using the scale I try and take the measurement as close to the centre line as where the load-cell would be , agreed as I take the same measurement closer to the gears using the scale the force required to open the probes increase.

I set the springs using a threaded bar and nuts until I have the correct force/weight whether using scale or load-cell, (roughly making sure each spring is compressed to the same amount).

Further , the old design with a cam/wedge used to seperate the arms and no gearing rest of design is exactly the same, the springs could be set with a scale over the centre line at 4.5kg force/weight which agrees with the load-cell at the same point. The new design requires the scale to be set to 10.2 Kg over the centreline where the load-cell would be but when I put the loadcell in it reads 4.5Kg between the probes

Do you think that the gears could be working against each other in some way >

I will find out the specs for the spring etc and take the measurement you mentioned

I appreciate your help guys

 

[desertfox]

Hi confused290181

Have a look at the uploaded file, it will show you how I calculated the force at the loadcell end using levers.
I see two problems with your set up, the first is setting the springs accurately because depending on the class of spring, they have different manufacturing tolerances which can alter both the spring stiffness and the load at a given compressed length.
Secondly when you set the spring, any attempt to pull the arms apart will compress the spring further and depending on how far the spring is compressed, that will increase the load required to pull the arm further and further apart.

Hope this helps


desertfox

 

[handleman]

Do you have any mechanical engineers at your company?

Before you geared the arms together, if you pull on one arm the only force you are pulling against is the spring. Just once. The other arm (I assume) was just held stationary by a hard-stop or something.

After you geared the arms together, when you pull one arm away from the other one, the other arm also has to move. The force that was at the tip of the arm due to the other arm is removed. That force has to be replaced somehow. It is replaced by traveling up the arm, through the gear train, down the other arm, and into your fish scale. You end up with double the force, plus some for friction.



-handleman, CSWP (The new, easy test)

 

[confused290181]

Handleman and desertfox,

Thank you very much , I think that eventually I was able to convey the problem to give you experts a chance of helping me,

I understand your point about the mechanical engineers at my company, however someone is going to have to swallow some pride on this one so I wanted to get my facts straight before I raised it, so its not just swept under the carpet by the designers with the calcs you have given me I will be able to not only show Whats going on but Why .

I think its remarkable that people like you spend their time to answer questions and do sketches etc I am truly thankful

Cheers

 

[desertfox]

Hi confused290181

Your welcome


desertfox

 

[confused290181]

One thing desert fox,

From your sketch where do you derive 9.81 from is that the spring constant or something

Cheers

 

[confused290181]

Just realised its a conversion factor my bad

 

[desertfox]

Hi confused290181

Make sure you do fully understand whats happening before you take it up with the designers,if you need anymore help just shout otherwise goodluck


desertfox

 

[zekeman]

Very smply, your gears are engaging a rack on either side and when the thing closes the rack and pinions are still engaged and very much contribute to additional forces, and the forces are probably not repeatable The cam arrangement on the other hand contributed no force after it's job was done.
If your designers insist on gears ( and I don't know why they would change to gears), then ask them to use a clutch on the driving gear so that once set the torques on the gears are removed by declutching.
Still puzzled why cams were changed

Puzzled