Counterweights for heavy lenses on vertical rails?

[Lou Jost]

I have some very heavy lenses, some more than 10 kg. This is far above the rated vertical capacities of most motorized rails. One idea I've had is to use a counterweight to balance the weight of the lens, like elevators use. Has anyone tried this? Any advice?

[ray_parkhurst]

You might consider an "idler" rail instead of adding more weight. A small linear rail can be mounted to the surface your rail mounts to, and a longer mounting plate fabricated that bridges between the current rail and the idler. This will greatly reduce the torque on the current rail.

[rjlittlefield]

Do you have to move the lens? This seems like a great situation to step focus by moving the subject.

If you do want to move the lens, does your setup have to be vertical? Even with a very heavy lens, the movement forces in a horizontal setup would be small.

If you do want to move the lens, in a vertical setup, then the simulator in my head says that a combination of idler rail and counterweight with an overhead pulley would give good stability and small load on the stepper.

--Rik

[elf]

You might consider an "idler" rail instead of adding more weight. A small linear rail can be mounted to the surface your rail mounts to, and a longer mounting plate fabricated that bridges between the current rail and the idler. This will greatly reduce the torque on the current rail.

How will this reduce the force required to move the lens up and down?

[Lou Jost]

Thanks for the suggestions. My subjects are preserved orchid specimens in alcohol, so the lens should be vertical. The subject moves slightly when the whole receptacle is moved, which is why I need to move the lens.

As an alternative, I could move just the camera. M=5-10 for these, and their depth is about 5mm, and the goal was to stack-and-stitch.

Another alternative is to use a front-surface mirror at 45 degrees, keeping the lens horizontal and the subject vertical. I bought lambda/20 mirrors for this, but the working distance between lens and subject is 10mm and that's not enough space for the mirror.

A third alternative is a horizontal main lens plus a horizontal relay lens (I'm thinking the PN105A) plus mirror, to increase my working distance enough to be able to use the mirror.

[ray_parkhurst]

You might consider an "idler" rail instead of adding more weight. A small linear rail can be mounted to the surface your rail mounts to, and a longer mounting plate fabricated that bridges between the current rail and the idler. This will greatly reduce the torque on the current rail.

How will this reduce the force required to move the lens up and down?

The usual problem with the rails is not weight per se but torque. Torque will be the limitation long before weight causes a problem. Heavy lenses are also usually fairly large, so torque increases faster than weight.

The Stackshot is rated to 10lb vertical, but they don't rate the torque, so I suspect this spec is a combination of weight and torque. Getting rid of the torque will probably increase the weight limit dramatically.

[rjlittlefield]

A third alternative is a horizontal main lens plus a horizontal relay lens (I'm thinking the PN105A) plus mirror, to increase my working distance enough to be able to use the mirror.

I'm guessing that your big heavy lenses are wide-aperture beasts.

In that case remember that any relay lens will stop down the system to the smallest effective aperture of any stage. For the PN105A, I'm thinking that's effective f/5.6 at 1:1, so as a relay lens next to subject it would essentially stop down your taking lens to NA 0.09 (=1/(2*5.6)) .

--Rik

[Lou Jost]

Rik, the main lenses have an NA of 0.28 to 0.35 and an m of 5x-10x. So they have EA around 7 or higher. So I think the PN won't be the restricting aperture, right?

Edit--these lenses are special for their FOV. The image size is more than 100mm x 100mm, at 10x.

[rjlittlefield]

Rik, the main lenses have an NA of 0.28 to 0.35 and an m of 5x-10x. So they have EA around 7 or higher. So I think the PN won't be the restricting aperture, right?

That would be true if you were using the PN as a relay behind the main lens, using it as sort of a 1X projection eyepiece for a 5-10X objective.

But as I understand it, you're proposing to put the PN in front of the main lens. That amounts to using the PN as a 1X NA 0.09 main objective, followed by a 5-10X projection eyepiece. Completely different ballgame.

--Rik

[Lou Jost]

Hmm, then the best remaining solution may be to move the camera and leave the lens stationary. Would that work at 5-10x over subject depths of about 5mm?

[rjlittlefield]

Would that work at 5-10x over subject depths of about 5mm?

I don't know. That amounts to dragging the lens away from its design point, so it's anybody's guess how well the image holds up.

--Rik

[Chris S.]

Lou, Velmex Bislide rails should handle these loads easily. I have a couple of BiSlides, and am impressed with them. (I quite like the cheaper, more common UniSlide rails for horizontal work, but would recommend BiSlides for vertical use.) According to the manual, "BiSlides have a load capacity of 300 lbs. (136 kg.) horizontally and 100 lbs. (45.4 kg.) vertically."

You might consider an "idler" rail instead of adding more weight. A small linear rail can be mounted to the surface your rail mounts to, and a longer mounting plate fabricated that bridges between the current rail and the idler. This will greatly reduce the torque on the current rail.

Ray, your general idea makes sense to me, but I can’t picture exactly how one would implement it. Can you elaborate?

--Chris S.

[Lou Jost]

That's a fascinating device, Chris. Out of my price range for now though. How is it controlled? Could the WeMacro knob-turning accessory run the manual version?

[ray_parkhurst]

You might consider an "idler" rail instead of adding more weight. A small linear rail can be mounted to the surface your rail mounts to, and a longer mounting plate fabricated that bridges between the current rail and the idler. This will greatly reduce the torque on the current rail.

Ray, your general idea makes sense to me, but I can’t picture exactly how one would implement it. Can you elaborate?--Chris S.

Please excuse the quick and badly-scaled drawing but this should convey the idea. I included idlers for both the rail and the camera/lens. I would assume the heavy lens has a mount of some kind that can be utilized.

The idler rail below the stepper rail reduces the stepper rail torque.

The idler rail the lens mounts to reduces the bellows mount torque.

The extended mount would have mounting holes along its length to give flexible placements for the camera and lens rail mounting to allow a range of magnifications.



edited to add: The idler rails can be most any type. Here is a carriage type, which can give additional rotational stability:

http://www.ebay.com/itm/MGN12H-CNC-Mini ... BN7cuqOFZQ

And here is a rod type, which can only give stability normal to the shaft axis:

http://www.ebay.com/itm/2x-8mm-400mm-Li ... SwYaxZkQqP

edited to add:

It might be better to put the stepper rail at the bottom, and idler at the top This way the stepper could be used alone if desired. But the long extended mount gives a lot of flexibility either way.

[Chris S.]

That's a fascinating device, Chris. Out of my price range for now though. How is it controlled? Could the WeMacro knob-turning accessory run the manual version?

The prices new are too rich for me as well. But decent buys come up on the used market from time to time. EBay item 391907526112 seems attractive.

As for controlling them, Velmex has controllers, as included in the example above, but I know nothing about them. (I have a similar, full-featured controller from another vendor, and after playing a bit with it—and its dedicated software/language—decided to get prefer Arduinos and their ilk, where my time invested in learning would give me more autonomy to build exactly the controls I want.)

However, the motors on my units are both easily-driven bipolar stepping motors. It would be trivial to terminate them to work with the Cognisys controller I use for focus stacking. (As my particular interest in BiSlides is not focus stacking, but positioning a microscope focus block prior to focus stacking with the Cognisys controller, I haven’t gone this route.) I have no experience with the WeMacro controller, but presume it would be equally easy to terminate these motors to work with WeMacro.

Also, these motors are NEMA standard sizes, so swapping motors should be pretty easy.

Please excuse the quick and badly-scaled drawing but this should convey the idea.

Your drawing conveys the approach very well! Thanks for this.

So the idler rails constrain motion to a single axis of translation, leaving the focus-stacking rail to perform only one job: provide motive force within that axis. Elegant!

--Chris S.