What type of screw thread has no wiggle or backlash? - Page 1
Forum section: electronics / general

author: NY2KW
author_rank: Regular Contributor
timestamp: May 05, 2025, 10:51:40 pm
content: I built a tripod platform for astrophotography that will support about 50-70 lbs of telescope and accessories. For general movement it has 10" pneumatic wheels but I want to build a self-adjusting leveling system with a screw rod for each corner. The leveling sensors, micr0controller, steppers are not the difficult part. The hardest part is finding threaded rods and matching flange nuts that have no wiggle . I thought ACME threaded rods such as used in 3D printers would work, but with a single point of attachment using a single flange nut for each rod, the rod still has lots of wigle. When 3 were attached to the tripod platform corners and brought down to ground, the platform has significant amount of wiggle even with light winds. I upgraded to larger diameter ACME rod (3/4") but no difference, it still has a lot of wiggle inside its mating flange nut. Attached is a picture of the platform with a telescope tripod and the upgraded rod and flange.ny suggestions for a very stable threaded rod system that would work?

author: edpalmer42
author_rank: Super Contributor
timestamp: May 05, 2025, 11:11:52 pm
content: Have you looked here -->https://www.wmberg.com/. If there's nothing there then ......In general, threads have tolerances just like everything else. Everyone wants to make sure that their standard threads will mate with everyone else's. That guarantees that there will be slop. The alternative is to find a machine shop or a hobby machinist that can make you a set of precision components. I've heard the term "machinist fit" used to describe a fit that has essentially no slop or wiggle.How are your flange nuts attached to the platform and what is the platform made of? You might need to beef up the platform and/or the flange nuts to get the rigidity you want.

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2025, 11:22:42 pm
content: For precision linear motions, we always used ball screw mechanisms (free of backlash):https://www.mcmaster.com/products/ball-screws/component~ball-screw/However, to avoid "wiggle", we supported the moving object with rails.https://www.mcmaster.com/products/ball-screws/guide-rail-carriages-1~/

author: thm_w
author_rank: Super Contributor
timestamp: May 06, 2025, 12:25:45 am
content: Those are thin rods, connected to a large flat platform, the force from the wind will be significant.You're probably going to have to rethink the design if you want a more stable base.Maybe the platform can be raised up then dropped onto solid concrete blocks or similar.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2025, 01:17:43 am
content: Linear screws (of whatever type) are not designed to resist side force, and if you constrain them sufficiently to do so you are likely to run into issues with them binding. If I were you, i wouldn't start from here!However you've already got the hardware that would be difficult to re-purpose, so it would be worth trying a low cost solution to see if you can get enough rigidity by adding a lower nut on each screw, level with the wheel axles, able to move slightly vertically so it doesn't bind, but braced with two diagonal struts to the center of each adjacent side of the platform. The struts should attach to the platform and nut carrier block with hinges or pivots with a horizontal axis at right angles to the edge, and the axis of the hinges or pivots at the nut end, if extended should ideally pass through the center of the screw, or at least come as close as possible to it. Either the struts should have a small length adjustment or the pivots or hinges at the platform end should be fitted last, taking care each threaded rod is normal to the platform, not forced out of plumb.In addition, you need to redesign the feet on the ends of the threaded rods to reduce their height and allow each foot to tilt without applying a bending force to the rod. To improve stiffness and reduce the risk of bending the screws, use wide blocks on the ground under the feet to minimise the distance the rod must extend below the (new) lower nut. You could with advantage use ball shaped foot ends on the rods, locating in hemispherical recesses in the top of each block to accommodate uneven surfaces that tilt the block.With approx. 20 lb force loading each rod, you don't have to worry about backlash!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 06, 2025, 09:48:01 am
content: Quote from: Ian.M on May 06, 2025, 01:17:43 amIndeed. You can also turn the whole thing around. In the link below you find workbenches with retractable wheels. I.e, the workbench itself is stable, and the wobbly bits are only used during moving.And there are many more variants of this.https://duckduckgo.com/?q=workbench+retractable+wheel+diy&t=h_&iar=images

author: jpanhalt
author_rank: Super Contributor
timestamp: May 06, 2025, 12:25:12 pm
content: I would consider making the legs permanent and casters retractable. For the legs 2" sq. tubing w/ 4" sq. pads would probably suffice. The casters (perhaps a little smaller) would retract up. They could retract onto or into the legs or just be attached to the platform edge and swing from above to below the edge for mobility. Similar designs are used frequently where stability is required, e.g., scaffolding and gantry cranes. Attached is an example from Amazon for a workbench.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2025, 02:06:53 pm
content: The O.P. wants to do auto-levelling, so retractable castors wont help much. OTOH it would then only need two short travel screw jacks as the third corner could then be a fixed landing pad, so retractable casters may still be worth doing.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 06, 2025, 02:51:46 pm
content: You can't level something on a wobbly platform very easily. First priority is a stable platform on the ground. Second is leveling. When using a transit, the platform comes first. The leveling is done at the head.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 06, 2025, 03:31:37 pm
content: Have you calculated the bending and the deflection of your thin screws when you apply a moment on the handle ?

author: tatel
author_rank: Frequent Contributor
timestamp: May 06, 2025, 04:23:55 pm
content: To be backlash-free, that acme screw/linear thing/whatever would need a second nut, and there should be some peloading between both nuts.Nothing new, it's usually called a "double nut"? It's usual on CNC lathes, milling machines, etc, and you should be able to purchase these double nuts, price being the only inconvenient part. That is really backlash free, if preload is well adjusted.Beware, today, linear movement mechanisms in CNC machine tools usually are not acme screws, thread does have an arc section, not a square section. Kind of what you could see in a gotic church, There are balls, not threads, in the nut(s), to reduce friction.You could anyway use acme thread like it was done in the past. Particularlly if you don't need any fast movement. Friction would also help to maintain the load in the desired position, otherwise you would perhaps need some means to lock it all. In that case you'd probably want to have just one nut, with that "machinist fit" mentioned in a previous post. That would have minimal backlash without being entirely backlash free. It wouldn't be as good, but would probably be cheaper, and good enough.If all you need to do is just two or three adjustements all along one nigth, even the usual threaded rod with nuts and locknuts could be enough?I'm just guessing your screws/whatever are able to support the load without bending, you should check that's the case.

author: langwadt
author_rank: Super Contributor
timestamp: May 06, 2025, 04:32:02 pm
content: Quote from: tatel on May 06, 2025, 04:23:55 pmit doesn't need to be backlash free, gravity makes the load always in the same direction, it needs support

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 06, 2025, 05:43:30 pm
content: Quote from: langwadt on May 06, 2025, 04:32:02 pmThey are related. The solution should be straight forward but fiddly. He seems to be looking to control angular motion of the rods. The allowed angular motion will be just geometry from the fit of the nut and the LENGTH of the nut. While specifying tighter fit nuts will help, the short length of typical nuts will still allow a lot of motion. But spacing two nuts many centimeters apart can readily reduce it by the ratio of separation to nut thickness. The fiddly part comes from registering the two nuts. Doing this "right" can reduce backlash further reducing the wobble

author: langwadt
author_rank: Super Contributor
timestamp: May 06, 2025, 07:37:43 pm
content: Quote from: CatalinaWOW on May 06, 2025, 05:43:30 pmthe second nut doesn't have to be nut to support it, it can be a sleeve or similar like in many linear actuators

author: jpanhalt
author_rank: Super Contributor
timestamp: May 06, 2025, 08:46:53 pm
content: I think many of the comments are "lipstick on a pig." The basic approach is wrong. Every telescope is based on having a solid foundation first. Get that done. Then deal with auto leveling.

author: coppercone2
author_rank: Super Contributor
timestamp: May 06, 2025, 09:09:59 pm
content: no, it needs active leveling of EMT conduit with a big network of piezo elements and AI learning control algorithm coupled to a accelerometer network . Think modern designhttps://www.pi-usa.us/en/products/piezo-flexure-nanopositioners/piezo-motion-control-tutorial/tutorial-4-9four design teams1) power systems2) control/sensor systems3) AI team4) CNC cut MDF wood bracket team (itscheapgreat)The mechanical work is nearly finished, there is no need to reinvent the wheel, I envision the project cost to be $1Bnote : market as "solid state" solution, has no moving parts to wear outOne of those projects that someone started by stealing a pallet from the loading dock as a 'systems engineering platform' and got a raise for innovative cost savings, then handed over the project to some division.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 07, 2025, 07:01:21 am
content: Quote from: NY2KW on May 05, 2025, 10:51:40 pmWhat you've done is not bad at all, you want to make it work properly without too much modifications and for few bucks, you can ask

author: Ian.M
author_rank: Super Contributor
timestamp: May 07, 2025, 07:27:48 am
content: Its certainly got issues - in addition to the lateral rigidity problem caused by the slop in the ACME nut and its short length, and concerns about the screw's ability as a column to resist buckling, how is the proposed self-levelling mechanism going to drive the screws?The end of the screw is up in the air and must move up/down as the screw turns, so coupling that to a motor is going to be rather difficult. The alternative of spinning the nut is also problematic because you then need some way of stopping the screw rotating, which at best will require something like a square section tube round the upper part of the screw, with a sliding plug in it attached to the top of the screw, leaving you with an obstruction sticking up at each corner of the platform. The inherent problem is the use of the screw as the leg extending all the way down to the ground, rather than as a mechanism to extend a wider and thus stiffer telescopic or pivoted leg moving a shorter distance.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 07, 2025, 09:25:53 am
content: Quote from: Coordonnée_chromatique on May 07, 2025, 07:01:21 amI don't really agree with this. To me it looks as a typical example of someone who does his best, but does not have much experience with making things. And that's OK, everybody starts there. But fixing the wobbling threads would require a relatively big redesign.About the simplest, is to use something like car lifting stands:https://duckduckgo.com/?q=car+lifting+stand&t=h_&ia=images&iax=imagesOr just shove some wooden blocks or wedges under the platform. The red tripod has it's own leveling feet. A bit more elaborate would be to make something similar to the leveling feet you regularly find under a caravan or motorhome. These go from simple jacks (built into triangular feet for stability) to electric systems with auto leveling.But it all sounds a bit too complicated. This thing looks like it's light enough so one person can lift it. or at most, make it so the existing towing bar can be used as a lifting lever (possibly with an overcenter).

author: Gregg
author_rank: Super Contributor
timestamp: May 07, 2025, 09:22:09 pm
content: Sand bags are a cheap solution to vibration problems and can be configured in many ways. Also they can be transported separately, like a wheelbarrow. I personally have some made from the bottoms of denim jeans after the knees have worn through. Rope or strap loops can be stitched to them before filling with sand and rope or bungee maybe via a pulley can be used to keep a steady tension on the lead screw.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 08, 2025, 07:46:42 pm
content: Quote from: Doctorandus_P on May 07, 2025, 09:25:53 amHello, i'm sorry but perfection don't exist and a lot of solutions can work, i repeat that it is not bad at all.Now try to immagine the double triangulation of a race car, on the two sides one triangle is attached with bearings on the platform fixed foot, the two others extremities of the lower triangle are attached to the floor side of the screws and the two extremities of the upper triangle are attached to the top ot the screws (rotulated bearings everywhere).Do you see ?PS : the nuts of the plaform screws are fixed on drawer ball sliders

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 08, 2025, 07:53:28 pm
content: You can use two nuts together, very slighly tightened against each other and locked in place, this will completely remove any backlash.You can place a spring washer between the nuts to allow better adjustment.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: May 09, 2025, 04:52:12 am
content: Quote from: Coordonnée_chromatique on May 08, 2025, 07:46:42 pmThe idea of NY2KW is working.My modifications gives a perfecly stable and rigid structure.The "wiggles" of the screws are cancelled by the two stacked sliders nut support.Equation solved