Linear displacement between two supports

3D Design
1

Hi all.

I want to put two supports (aluminum) in motion in relation to each other. The goal is to put them in contact or not. The rest position of the system is "both supports apart" and the distance between the two supports is about 8 minutes max when the system is at rest and in contact when the system is at work. On my 3D, the blue part is fixed and the beige is mobile.

To do this, I put 4 axes associated with brass bearings to allow this movement. These pins have a large diameter to keep the parallelism of the two supports. Slots are provided for return springs that will move the supports apart when the system is at rest, these springs are optional depending on the actuator used.

Namely that I only have an electrical source to automate the movement, so the movement will be carried out by an electric actuator.  The effort to put the supports in contact is 13 Kg + the weight of the support, for this I plan to use an electric actuator which provides a force of 18kg.

Before thinking about the 3D provided in this thread,  I made on an aluminum profile model with axes and bearings bought on the net, but with a displacement of 35 mm instead of the current 8 mm. After hours of adjustments to align everything, the tests on the movement were far from optimal, so I come to try to find help.

I am looking for your advice to set up the solution, the most suitable or functional for this trip.

And here are my questions:

Do you think that the structure imagined on the 3D is correct?

During my 1stattempts , I used two cylinders of identical reference (thinking that they would have an identical speed of movement too), to push the support from the center of the two opposite widths so as close as possible to the axes. As these cylinders did not have exactly the same speed of travel, after about 20 mm the displaced support was no longer perpendicular to the axes and the system jammed. I changed these two cylinders by another one (with 2 times more force) and I tried with this single cylinder to push the support from its center, the operation was better but still not enough to have a perfect and above all repetitive movement.

My conclusions about these problems are:

As the structure is made of aluminium profile, it is almost impossible to be sure that the thrust is applied to the centre of the support to be moved. I guess the efforts were not perfectly distributed, moreover the movement was jerky as if to "overcome" constraints on the axes.  

The actuator attachments were also fixed at both of these ends, perhaps the actuator was also adding stress to the axes during travel.

I imagine solving these problems:

By reducing the travel stroke.  8 mm instead of the starting 35 mm (the stroke of my cylinder is 40 mm)

By using the electric actuator in "single acting" mode (with return springs), so the actuator only pushes and it is not attached to the support to be moved, there should be less stress.

What do you think of the push point in the center of the support, so far from the axes? Is there a problem to consider?  

What do you think?

I had also imagined a kind of camshaft moved by the electric cylinder and the cams moving the aluminum support, but this is beyond my capabilities and will increase the number of parts to be made.

Solenoids (next to each axis) could also have done the trick for this short trip, but with the same risk that they don't move at the same time.

If you have any ideas to propose to me, remarks on my structure.. I'm a taker for everything!!

Thank you in advance.

 


assem1.igs
2

Hello Bernard, 

Yes, linear cams would have been the best solution from the point of view of precision, small footprint and above all allow the mechanism to operate with only an actuator, but hey,

Have you looked at the electronic synchronization module side?! See even make your circuit using servomotor, small stepper motor an arduino? 

Should the plates remain parallel in the rest position ? Otherwise you can place 2 hinges on one side, the cylinder on the other end, it would distribute the loads better and allows you to easily lift the 20kg ;) 

 

1 Like
3

Hello @bernard.bouffil ,

You are faced with the classic problem of translational guidance, namely the phenomena of chatter (stick-slip for our neighbours across the Channel) and buttress. Both phenomena originate from friction in the guidance, and the remote application of a motor force.

The first question to ask yourself concerns the number of "columns" to provide guidance. With two columns, the system is already hyperstatic, and it is even more so with four. Even if this arrangement gives a feeling of stability and equal contribution from everyone, it is not so, and it is impossible to know who is actually providing guidance, and who is supporting what effort.
You say it yourself: "After hours of adjustments to align everything... ". What happens to this alignment once the system is loaded?
As it is not possible to place a single guide in the center of the plates, I would test a solution with only two columns, in one of the diagonals of the assembly.

The second point is friction: it is the initiator of the negative phenomena described above. The only way to remedy this is to reduce it as much as possible. The use of ball bushings makes it possible to reduce the "coefficient" of friction to a very low value. Linear guide ball bushings can be found with inner and outer diameters of the same order of magnitude as plain bearings.
Disadvantage: they are longer than the bearings you have selected, and must be fully housed in a bore. Theoretically... This will lead to thickening the support, or using a sleeve.

Third point, motor effort. I assume that the assembly is placed horizontally, and that the load is due to gravity acting on the components of the movable plate. So a vertical force located in the vicinity of the center of the plateau... It should be noted that the motor is a motor for the upward direction of travel, and that it is the load that becomes a motor in the downward direction.
Ideally, the guidance should be carried out in the immediate vicinity of this force, as well as the action of the motor. I imagine that this arrangement in the middle of the board will not suit you. ;-)
Using two motors arranged symmetrically is a solution provided that they have identical displacements: stepper motors, or servo in position, as mentioned by Lynkoa15.
Similar solution from the point of view of effort: use a single engine exercising two actions via a feedback system. It complicates the mechanics, but it exists on some 3D printers...
A single-motor and single-action arrangement is possible, it will only be necessary to estimate the defects in the position of the mobile plate in relation to the fixed plate, due to play and deformation.

Back to the question of guidance: the manufacturers' recommendation for the use of ball bearings is to combine two bushings to achieve a "long" guide, of better quality. Is such an outgrowth conceivable on only one of the columns, especially in the area where the engine is located?

Choices to be made, and tests to come on your model. Good luck.

Kind regards.

1 Like
4

Hello and thank you for your answers

Lynkoa15, when I started thinking about this system, I wanted it to be at a low cost. So to avoid machining single parts, I chose to use an aluminum profile structure cut in the factory and to be assembled, to buy bearings, pins and linear ball bushings from the store, and cylinder with continuous motor (no stepper motor to imagine a synchronization). That was the 1st of my mistakes... because since then I have had several small machined parts made to correct the defects of the profile structure, which is starting to become expensive (the savings of ends of string often cost the price of the spool!!)

M.BLT

Indeed, the positioning of the assembly in operating mode is horizontal and the movement vertical. The purpose of the set is to lift and press on spring-loaded feelers, distributed in the holes visible on the blue plate.

The upward force is developed by the cylinder which consists of moving the moving part above (its weight) and crushing all the springs of the feelers, hence this short travel of displacement. 

The downward force is developed by 4 return springs next to the axes, the return springs of all the probes, the weight of the assembly. If necessary the cylinder being a double acting, it can also "pull"  

Yes, the two supports must be parallel, as the probes pass through the blue plate (see 3D)  to protect them in unpowered or resting mode. 

The choice of the 4 columns was made, as you say I imagined (by my false intuition) that it would allow a better movement based on the principle that the arrangement of the probes is almost homogeneous on the plate to be moved. I'm going to try to use only 2 columns as you say diagonally.

In the 1st structure, I had used linear ball bushings , it didn't work any better, maybe more because of the accumulations of alignments caused by the structure of aluminum profiles than ball bushings . But no , we noticed with a mechanic a relatively large play of the support between the axes and these bearings even though they are adapted (these bushings seem to be more suitable for horizontal displacements that rest on the bushing..??).  

On the advice of this mechanic, we started to optimize the system while keeping the aluminum profile structure (so it was a bit of a patch to try to use the existing one). He machined brass bearings adapted to the axes, made parts to replace the commercial bearings fixed and adjusted on the profile structure.  This made it possible to have two sets (one per width of the support) that flowed perfectly (but only on two columns per set). The profile structure does not allow the opposing sub-assemblies to be made solid. All that remained was to adjust these two opposite sides and after a lot of adjustment time, all these modifications improved the movement to be almost satisfied.

I was disillusioned when I realized that just the tightening of a screw, a lesser shock or less stress on the structure of the aluminum profile was enough for the system to start jerking again. 

Hence my choice to make a new completely autonomous assembly. The upper part will only be suspended on the aluminum profile structure in order to avoid the constraints due to the latter.

I provided another 3D, can you tell me what are the problems that jump out at you?

I switched to 2 axes (columns).

The force is transmitted via a 2.5 mm thick bent steel part to have more rigidity than with a solid aluminum part (cross piece). 

Should the cross-shaped piece press on the 4 corners of the moving part or only on the angles where the axes (column) are? 

The cylinder rests on the center of the cross in a blind hole that guides the cylinder head but without rigid fixing to avoid stress on the columns.

Thank you

 


assem2.igs
5

Hello @bernard.bouffil  and @ Tous

Because the movable and fixed plate is 361 x 200 mm, the problems of buttresses are formidable as our colleagues immediately indicated.

Personally, I think that the solution with circulating ball sockets, in addition to the difficulty of implementation and its size, will not satisfy you.

In the past, faced with almost the same problem for micro-machining, I opted for two solutions. Note that the following is reserved for weak races.

I leave aside the compliant systems which often have too little travel.

The first solution is a caged eccentric.

The four eccentrics are synchronized by axes on the 200 mm dimension and axes and bars on the 360 dimension. Disadvantage: the travel is limited depending on the size. Advantages: high accuracy and repeatability on the dimension in the high and low position and even in any intermediate position.

The second solution is knee pads synchronized by an eccentric roller or a knee pad.
Advantage: high precision at the high point and easy adjustment of the high point. (We can take inspiration from the particular knee pads of injection molding machines. Disadvantage: intermediate positions are not recommended.

If there is not enough space, then a system with corners (at all four corners) gives good results because its size is smaller than a knee brace and the rise is more gradual and requires less effort

You will notice that in the three solutions I propose the oscillations due to the springs in particular are totally controlled because the spacing of the two plates is always controlled, no play in the vertical and horizontal direction. It also solves the problems of buttress!

That's something to discuss, all together of course

Kind regards

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6

Hello@tous 

Known as Bernard, what type of probe? Because if it's the ones used in o0 metrology, we'll have to consider a micrometer  order accuracy as mentioned by zozo 

7

Hello again all,

Here are some remarks in response to your questions or suggestions and for a better understanding of my needs. 

Lynhoas15, the probes used (consisting of a barrel and a spring-loaded piston)  only allow a momentary electrical connection to be established between conductive surfaces that are located on a surface positioned in the fixed part of the system. I don't need precision, neither at the bottom, nor in the intermediate, it' s really 0/1 or high/low position. Only the high position is important and it is the probes that take care of the precision of the electrical contact with their respective pistons and springs.

This displacement can be broken down into two  :

  1. Start the upward movement with a moderate effort of 5 or 6 mm to establish just the contact. The effort to be made to this displacement is related to the weight and friction... of the moving part.
  2. Continue this upward movement between 1 and 1.5 mm to bind the springs included in each probe, the effort to be made is greater and is closer to 13kg, effort proportional to the chosen stroke, the closer you get to 1.5 mm the greater the effort.  This last move consists of increasing the quality of the electrical contact and/or overcome height differences between conductive surfaces (about 1/10 of a mm). 

Zozo_mp, it seems that you have been confronted with  this kind of problems, even if my need for them requires much less precision than your systems. I would like to understand your solutions, but unfortunately I will need more information, even sketches to understand them. I'm an electronics technician at the base and I lack vocabulary, what do you call "eccentric", "caged eccentric"...  

I also agree with you when you tell me about the size of this "press" 360 x 200 mm with a single central cylinder and the boom that will apply the support piece. Hence my steel design of this part, but even in steel the problem will exist? 

 

My real problem is this displacement of a few mm and these buttresses.

I also thought of 4 worm screws (or similar) next to each axis, they would be synchronized by a belt or a chain in order to rotate these 4 screws together either with a motor, or with the current cylinder to move the courroir and therefore the moving part. 

  1. Is this complicated to implement?
  2. Maybe this will solve my current travel problem ?
  3. Are there similar solutions that are easier to implement?

Thank you for your advice, your time and I'm always open to ideas.

Bernard.

 

 

 

8

@bernard.bouffil 

I'll make you the proposals tomorrow.

To pluche

9

Hello 

Great and thank you for your time. I am waiting for your return.

Bernard.

10

Hello 

On my side, Bernard, if you think you can get away with the screw-belt system, it is because the cam mechanism is at your capacity (less complicated and less expensive), here is an approach,

Mr. Belt and concerning heperstaticity, I don't quite agree with you, I've already seen designs with several guides (similar to the attached images stitched on the net), they were blocks about 1m punching-stamping fine sheet metal (I think 2/10) so with a relatively tight die set, Also by quickly looking at some references on the net, the kind of socket described accepts an operating clearance of up to 120 microns, more than enough to compensate for precision manufacturing errors in the industrial context, even for amateurs, I noticed while making my CNC, the structure and type of profile used are so light that their elasticity compensates for the misalignment :). 

 


screenshot_20220819_094124_com.hsv_.freeadblockerbrowser.png
assem1.avi
2 Likes
11

hello Lynkoa15,

Far be it from me to think that I can get away with screws and straps. I just think about everything, it's after I thought about the cams. But unlike electronics (my job) where modifications are "easily" achievable on my kitchen table, in mechanics it's much more complicated too, especially since I have to have the parts made every time I think I've found a solution. Hence my concern about my reflections!. Laughing out loud

I'm going to look at your solution, which is a cam principle it seems to me, but simpler than the ones I've thought of.

I continue to look and thank you for your help and advice.

Bernard.

   

1 Like
12

Hello everyone ,

Attached is a new 3D based on Lynkoa15's proposal. With this 3D comes of course questions.

On this assembly, the cylinder does not directly apply its "force" about 18 kg on the moving part (in beige). Can we consider that in this last assembly the "force" applied by the cylinder through the cam part  (in blue) is almost identical (18kg)?  or more? or less?.

The parts must be made of aluminum (which I would say standard). Do you think the dimensions are correct for the forces to be developed, or did I oversize them?

The 4 stops (in yellow) that move up and down depending on the position of the cams rub on the cam mount (in grey/blue). Should they be made of brass to be self-lubricated, of steel so as not to wear out, be a ball finger ...?

 I put flat ball bearings (in purple) to facilitate the lateral movement of the cam support (in grey/blue). Only one side of the bearing is mounted so that the cam mount is in contact with the balls. Maybe just a sheet of plastic, epoxy can suffice. The same question arises for the openings of the corridors of the axes. I'm asking myself too many questions and alu against alu will work just as well? Namely, that the system will have to work about 700 times? 

I tried to create a dynamic, but I didn't know how to use the constraints to make the stops (in yellow) and the cams follow. Is this possible? 

On my 3D, the two holes on the axis of the cylinder represent the "exit" and "in" positions. 

Here again this time, I hope to have some remarks from your to avoid useless fabrications.

 Thank you

Bernard.


assemcame.igs
13

Hello

A few thoughts in the attached document to clarify my views on your system design.

I have just discovered your proposal on the basis of a movable frame with inclined ramps.
In principle, the solution suits me and could work...
A few remarks though:
- Ball stops are not suitable for translational movement since the center of each ball has a circular path. There are linear pads for translation, but a simple washer with an anti-friction coating should be fine;
- the pushers must be lengthened so that they can go down the ramp ;-)
- the theoretical action of the cylinder depends on the slope of the ramps.
If we denote the angle with the horizontal     plane alfa: Fvérin = Load * tan (esparto)   without friction.
With friction at the ramp/pusher contact: Fvérin = Load * tan (esparto + phi)    where phi is the angle of friction of the material torque (Usual value for metals: 8 to 10°).
Reduce the overhang of the pusher, again to avoid buttress, or at least limit friction in its guide.

- Contact between aluminum alloy parts is quickly prone to galling.

Have a good studious evening...

 


supports.pdf
1 Like
14

Good evening

Mr. Blt, thank you for your literature and the time spent.

For the ball stops , I had a doubt but I thought that the balls "rolled" in all directions in their casing. I'm going to replace them with a Teflon washer, it seems to me that it's self-lubricating and will do the same thing on the sides of the oblong windows through which the pins pass. This should eliminate metal/metal friction.

For the angular calculation of the loads I will see this with a clear head!. But I suspect that the longer and gentler the slope, the better it works. In my I am related to the size where this system should be, I cannot have a displacement greater than 40 mm (horizontally) so a slope of 15° that I can reduce a little by reducing the amplitude of my vertical displacement.

For the pushers you tell me that they are too short, yet I see them at the right length lol? maybe I misunderstood the system? Depending on the position of the cylinder, when the finger is at the bottom of the ramp I am in the rest position (probe support at the bottom) and when the finger is at the top of the ramp I am in the working position (probe support at the top), I provide the two images in both positions.

In Img1.png: the cylinder is retracted and the part with the slopes (blue/grey) is as far to the left as possible. The finger is on the upper part of the slope, and pushes the moving part vertically (in blue) upwards. A probe in the working position is seen protruding from the purple part.

In Img2.png: the cylinder is out and the part with the slopes (blue/grey) is as far to the right as possible. The finger is on the lower part of the slope, the springs of the slope, the gravity..  Brings the moving part vertically (in blue) downwards. The probe can no longer be seen, which is in a resting position and protected under the purple piece.

In my system, unlike you, my pushers are solitary from the moving part vertically (in blue), yours seem to go through it. You are talking about the pusher guides , currently the horizontal pusher guides are only made by the 4 columns. Should I consider a "groove" to guide the pushers horizontally?  

The last friction left is the finger for horizontal movement, I thought  of putting a ball pusher (a screw equipped with a ball on spring at the end) is it intended for this kind of application or should I cover the slope with a non-abrasive material? 

well there are enough for this Sunday lol  your advice and ideas are moving this personal project forward . 

Thank you and see you soon for your comments if time permits. 

Bernard.


img1.png
img2.png
15

Mea culpa, mea maxima culpa...

I plead guilty, I was not attentive enough: by focusing too much on the ramps and pushers, I got it into my head that it was the upper plate that was mobile in relation to the fixed lower plate.
Of course, it's the other way around, so the pushers are quite long.

16

@bernard.bouffil 

Can you give me some information so that I can make you the promised proposals.

  1. What is the maximum height of your system.
  2. What is the max force when all the springs are compressed
    3. You mentioned cylinders, can you confirm whether electric or pneumatic cylinder.
    4. Can you confirm  the number of movements per minute
    5. You talk about an 8mm stroke without saying how you insert the parts between each sequence

I ask the question of the size because I think that ball bearing columns are not suitable because of the overall volume.

Kind regards

1 Like
17

Hello

Just a small remark about wear, I would avoid edges that are too sharp for the "slopes", especially the one at the top of the slope, particularly abrasive when going up with the finger. I would add a leave to create a smooth transition from slope to flat.

Adding one at the bottom of the slope, even without abrasion problems, would have the merit of fluidifying the ascent movement, the engagement on the slope, which would also reduce the impact of friction on the edges of the finger, and therefore wear.

(NB: I just hit by zooming in that @m.blt did like this on his image)

For the finger, it is related to the guidance, and also to the friction. Maybe 2 ideas to consider: "beam" fingers, square instead of cylindrical, shape that would guide itself. Then rubber capstans (the kind you find in the late Walkmans to press the band) rather than ball bearings (well they are ball bearings but surrounded by rubber). The latter would have the advantage of no longer being subject to abrasion of the slope edges, thus avoiding making leaves.

1 Like
18

Hello and thank you both for your help.

Zozo_mp

For a little more information: this system will allow me to do electrical tests on a printed circuit board. The latter is guided and positioned manually on the 3D part called plexiglass. The displacement we are talking about makes it possible to make electrical contact between the spring probes and a printed circuit board. The whole thing is in a structure that integrates electronics, a little management of the whole thing and a cover so you don't have to pinch your fingers.

1. What is the maximum height of your system?

 We have everything included 200 mm in height. Note: the total volume (LxW) is defined by the part named "Plexi plate", you must have 12 mm free inside all sides of this part, see my 3D

2 What is the max force when all the springs are compressed. 

There can be up to 120 probes that require 1.27 N each to be fully depressed, but the system will still be functional if they are only 70% depressed , within 0.9 N. So between the max force is 155N but functional at under 100 N.

3. You mentioned cylinders, can you confirm whether electric or pneumatic cylinder.

I don't have a pneumatic source on hand so yes it's an electric cylinder that develops a force of 180N with a stroke of 50 mm 

4. Can you confirm  the number of movements per minute.

We don't count by the minute. A movement about every 3 minutes, but time is really not a constraint. It is the time of insertion and processing of the IC that takes time. 

5. You talk about an 8mm stroke without saying how you insert the parts between each sequence. 

Setting up the PCB is manual and takes much longer than testing. I estimate the time at 1.50 minutes for the installation and removal of the printed circuit board, and the actions around the pcb.

Syll, I reworked my 3D. I brought the axes (or columns) closer together on the long sides. This allowed me to use the entire stroke of my cylinder (50 mm), and therefore to lengthen the slope and have a softer angle. I also thought about putting leave to avoid salient angles.

As for the various rubbings, here are my thoughts:

For the friction between the underside of the horizontally moving part (the one with the slopes) and the shoulder at the bottom of the axes (columns), I am looking for and thinking of putting a Teflon washer or other self-lubricated material, the aluminum parts will "slide" on this washer.

For the friction of the axes (columns) in the slotted holes of the horizontally moving part (the one with the slopes), I think to widen the opening to avoid friction on the sides of the oblongs. But this implies that the fingers must become guides and these are the ones that will avoid the friction of the axes. I also added 2 fingers 6 instead of 4 because I had to reduce their diameter 

For finger rubbing in throats and on slopes, I think I can do them directly with ACETAL POM C. 

What do you think? Another idea, why not make the piece directly with the ACETAL POM C slopes? Will it be rigid enough? not 10 times more expensive.

See you soon

Bernard.


assemcame.sldasm
19

The problem with aluminum is that if it is not perfectly polished, it will be abrasive. If polishing the surface is not too problematic, polishing the slopes/ramps will be a big hassle, in terms of average cost and time, so indeed it is preferable to make the part directly in Acetal pom C. Even if it means stiffening it with an aluminum frame/bed, which will not need any particular surface treatment.

In terms of finishing, there is also the machining method of the parts to be taken into account; milling? moulding?

PS: the joint assembly is illegible, you have to create a PackAndGo.

1 Like
20

Hello

I have to compare the prices between aluminum and ACETAL.

I repost my 3D to the right format

The grey frame represents the area to be left empty. 

See you again.

 


assemcame.igs