Hi Bob, I know exactly what you're talking about. I had a similar problem with my Dob mount when I was still using it. I know Teflon has a very small static friction coefficient, but I'm not sure about melamine. My Dob mount used Teflon pads and a linoleum base for it's bearing surfaces. The pads were too big and there were only three. I changed out the large pads with much smaller pads, but added five more. I mounted them in a staggered pattern that formed a small circle closer to the shaft and a larger circle near the edge. Three pads were used for the smaller "circle" and five pads formed the larger outer "circle". This helped for several reasons. The weight was better distributed, the total surface area that was is contact with the linoleum was decreased, and due to added stability, binding in the shaft assembly was also reduced.
Keep on Gazing! (33N X 112W Phoenix, AZ - Clear Sky Chart)
Well I have 3 pcs 25x25 mm Teflon pads on the bottom of the two circular plates.
They are located about 25-30 mm from the outside edge at 120 deg separation. The diameter of the plates is 400 mm.
Are you suggesting that I could reduce the size of the pads and put more of them instead?I always assumed that no matter how one did these things a maximum of 3 support pads would actually carry the weight (the highest 3). How can this work?
Another thing: will the stickiness reduce if the Teflon surface is increased or decreased? What is the physical property involved here?
In the Stellafane description they cover the base of the mount with a material called "Ebony Star" kitchen top laminate that is found to work well with Teflon. I just wonder how I can get hold of such....Here is the Stellafane bearing page.
I probably will have to look for ways to get the laminate to put on the bottom surface of the mount....
Bo BSweden(N59d24.27' E17d49.12')
Bob, I only know that it worked for my Dob mount. I adjusted the mount so that all the pads made contact, but then I have a machine shop available to me, so that wasn't too difficult. I know also that there are various types of melamine laminate that vary in smoothness, so I really don't know what you are working with. The physical property involved here is the static friction of the pads/melamine and the shaft. I forgot to add that I also added a thrust bearing to my shaft assembly as well. With the combination of the smaller pads, less contact surface area, more stability and the thrust bearing, the total static friction was reduced far below its original value and the mount rotated with very little "stickiness". I'm not sure if you're familiar with static and kinetic friction, but they are culprit of your problems. They are what cause the mount to take more force to get moving (static friction) and once moving (kinetic friction) being much less than the static friction makes the mount much easier to move and it therefore overshoots. My fix was simply a suggestion, but one way or the other you need to reduce the static friction. For all I know, Teflon against "Ebony Star" has a very low static coefficient and may solve your problem, but having never worked with the latter I just don't know.
Thanks for your suggestions!This is my first telescope project and I did not find the Stellafane site until I had already started...
Anyway, it looks like I have to do something about the surface of the bearing since the melamine is probably too sticky. I have found a US supplier (Meridian Telescopes) who sell some similar material for bearing construction so I will probably order that and see what gives. My boards came with a melamine surface treatment that I thought may work, but it is now obvious it didn't. I have seen strange bearings in other sites, like old used 12" LP records....
Concerning the center bearing, I have just drilled a hole straight through both disks and hammered in a Tee Nut for the bolt in the bottom part. Then I run an 8 mm bolt through.
Can you describe what you did to add the "thrust bearing"? What does that mean?
Bob, first let me say that I tend to overkill most projects I work on. I even installed radial and thrust bearings on my Tasco EQ-2 mount. In simple terms, a thrust bearing has it's load distributed parallel to the shaft, where as a radial bearing's load is perpendicular to the shaft. So for the thrust bearing installation, I machined a shaft from stainless steel and replaced the bolt with the shaft. I used a router to put a pocket in the upper part of the mount for the thrust bearing housing. The thrust bearing I used was a tapered roller bearing similar to the wheel bearings used with the rotor assembly on a car with disk brakes. That type of thrust bearing is really a hybrid with thrust and radial loading components to it. The bearing I used was a bad bearing from a CNC machine tool that I had repaired and though it wasn't any good for application on the the machine, it was fine for my application. The Teflon pad/melamine configuration is actually a thrust bearing configuration in itself, but has no radial component to it. The bearing really isn't carrying the full weight of the scope, the Teflon pads are. The thrust bearing does carry some of the load but it's main purpose is to keep the upper and lower Teflon/linoleum bearing surfaces parallel to one another - which helps to keep an equal amount of load on all eight pads.
Before I added the thrust bearing, I had modified the number and configuration of the pads as in my original reply, and it did work fine. The reason I added the thrust bearing later was due to occasional binding in the shaft assembly - that and I love to over engineer things. Because I didn't want to get too technical, I originally only mentioned the change in the pad configuration which does help. But you're right about having to get each pad adjusted to the same height so they all bear close to the same amount of load. If you don't then only the three "highest" pads will be carrying the load - as you previously mentioned.
Since I doubt you'll want to go to the trouble of installing a bearing, I'll make another suggestion that will help. Make sure to use a fender washer between the bolt head and the material it contacts. Don't over tighten the bolt as this will add friction to the assembly. To keep the bolt from loosening or tightening during mount rotation, use a little RTV (silicone adhesive) on the threads of the T-nut. The silicone will keep the bolt adjusted properly, but if you need to remove it later, you still can. If you used Thread-Lock (3M), it will be much more difficult to remove the bolt at a later time if you need to.
I hope I haven't confused you too much, Bob. If you still have questions, I'll be happy to answer them the best I can. If need be, I can also bring the technical level down a bit more.
Ed,I am probably not going to be able to make a bearing even though I can see the advantage of it now. I don't have the machining facilities at all, I only have an assortment of hand tools available to me...
By having a proper bearing that carries the bulk of the weight of the tube and upper part of the mount then there will be less force on the pads so they won't create as much drag as they do now. Basically if one could make the height of the bearing adjustable it should be possible to get most of the weight on it and then the pads would only be used as guides to stop the system from tilting.
Well, I have just ordered some "Glassboard laminate" from Meridian so I can replace the melamine glide surface with something with less friction. It will be sent to my daughter in Grand Rapids, Mi, where I will pick it up during our holiday visit starting Thursday. So I will not be able to work on this until next year.
Question:Would it not be possible to put a spring on the center bolt in such a way that it will lift the mount upper parts, or rather take away some of the force from the pads and thereby reduce the friction?Say using two washers with a spring in between on the bolt and then the mount upper part on top of the top washer.
This would move the bulk of the mount weight to a small radius and therefore make it easier to overcome by pushing on the scope...Which actually makes me wonder about the large radius bearings used in the Dob designs, would it not be better with small radius bearings so that the move force would not have to be so big???
Bo, the radius of the pad location is needed for stability as you surmised earlier in your post. Also, with all things being equal, the amount of friction would be the same regardless of the radial placement of the bearing contact points. It's a function of the amount of weight on the bearing surfaces and the static friction coefficient of the bearing materials. However, if you were to install a "handle" or lever (a wooden dowel would work fine) to the mount and used it to rotate the scope, that would lessen the amount of force you need to apply to rotate the scope - it would also give you much more control over the rotation. Generally speaking, the bearing placement (as far as radius is concerned) won't really change the amount friction, but the actual point at which the force is being applied will affect how much force need be applied. It might even solve your problem altogether, or at least make it manageable enough to still use the scope until you can change the melamine out. The further away from the center of rotation that a force is applied, the less force is needed to do the same amount of work. Perhaps this is what you were thinking of regarding the radius of the bearing placement. In physics, it's referred to as the "moment arm" and is the principle that levers operate on.As for the spring idea, that could help, but only with certain caveats. You would want to use hardened steel washers, a spring that has both ends ground flat, and a little grease for lubrication. This would help not because of the placement, but because of the low static friction coefficient of hardened steel against hardened steel. If you cant find those materials in Sweden, you may be able to find them here in the states when you come for the holidays. Ace Hardware usually has a good selection, though it sometimes depends upon the store as they are a franchise.
By the way, I just realized that it's Bo and not Bob. Sorry about that.
I used a large all metal lazy susan type bearing on my Dob and I end up having to tighten the center bolt as it moves too freely now
Hi Norm, that's one of the reasons I let most of the bearing load be carried by the Teflon pads rather than the thrust bearing. Using that type of bearing is a great idea by the way - even if you do need to introduce a little more friction to the system, that tends to be easier than having to remove friction.
Now back home from the US trip and I brought a sheet of glassboard laminate for testing.
I just made a quick test by dismantling the mount and putting the laminate between the teflon pads and the top part (no center screw just yet). Then I tested its friction and it turned out to be way lower than the original melamine surface with ArmorAll treatment. Especially noticeable was the "sticky" friction for starting the movement of the mount on pushing it. So it seems like this will be a big improvement!
My next problem is how to get rid of the ArmorAll treatment from the bottom of the moving mount so I can glue on the laminate without risking it coming loose later on...
Concerning Lazy Susan:
I did not at first know what you meant by this but after some googling I got the message...
Seems like one can get such a bearing quite reasonably priced up to a diameter of about 450 mm. Like this:
Diameters around 300 seem very inexpensive and even going up to 350-390 mm diameter does not mean a ruining expense either.
Would this be preferable over the laminate/teflon pad system?
I think the Armor All can be removed with any household spray cleaner. Gluing to melamine isn't easy as tthe surface doen't have any pores. Look into a product called Roo Glue also a good 2 sided tape will work, some of these tapes sold in wood working stores allow blocks to attach to lathes for turning bowls - very strong adhesion. I guess this info may not be needed if you go with the bearing set up which looks similar to the bearing used under my Z10.
Good luck Boberglund, hope the Armor All suggestion doesn't set you back.
EASY DUDE, THERE WILL BE ANOTHER WAVE
From San Diego, Zhumell Z10 (Bubba is for all around observing)Telrad, Vixen VMC110L on a Porta Mount II, Bushnell 10x50 (for beach) Garrett 10x50 classics (for Sylvia), Zhumell 80x20 on their Pro tripod(cuz they were cheap and sylvia digs 'em), 1.75 readers (so I can find the other stuff)
As Ken indicated, a good spray cleaner or alcohol should clean off the Armor All sufficiently, but as a last resort, acetone will work. If you do need to resort to acetone, however, don't pour it directly on the melamine. Instead, just moisten a paper towel and wipe the melamine clean. You'll probably need to use more than one paper towel since acetone evaporates so quickly and you don't want to recontaminate the surface with the residue that the paper towel picks up. Be careful not to use too much acetone as it may start to dissolve the surface of the melamine - it's also extremely flammable so work in a well ventilated area away from any ignition sources. Hopefully you won't need to resort to acetone, as the spray cleaner and/or alcohol should do the job. After cleaning the surface, I'd recommend using contact cement to adhere the glassboard laminate to the base.
Since you already have the new laminate and the test seemed to work well, I'd give it a try before spending anymore money. You can always resort to a "Lazy Susan" bearing at a later time if needed. It's really just a very large thrust bearing and would work very well, but as Norm indicated, it may turn too easy with that method and require that you introduce friction to the assembly. You can tighten the center bolt to add some friction, but over-tightening a bearing can cause intermittent "skidding" of the balls against the race. That's one of the reasons I used both Teflon pads and a thrust bearing.
In the end, however, it's really just a personal preference. Both methods should work very well. If it were me, I'd probably go with the "Lazy Susan" bearing and add whatever friction necessary. On the other hand, the laminate/Teflon pad system is less complicated and should do the trick. As we say in America, "six of one, half-dozen of the other." I guess it comes down to expense and how comfortable you are with work involved. Good luck and let us know how it works out.
Less friction!After cutting the laminate to size and drilling the center hole I could test it indoors and it sure is much less sticky than before and the movement starts at a much reduced force on the tube as well.
Mounting the laminateThen I also realized that I don't have to glue it to the bottom of the mount at all. I just have to staple it to the underside in a few places outside the ring that is actually going to rub against the Teflon pads. Much easier to do and possible to remove easily too. While in place it won't go away since it is locked between the disks.
Lazy Susan?I managed to locate suppliers of these bearings as I wrote above, but unfortunately not a single place in my neighborhood. All seem to be abroad actually. So I cannot make a test easily...But I am curious about the comment that it might have too little friction; what will be the effect of this? Will the scope be moved by winds or is it too easily moved by the face touching the eyepiece while watching?
Slow motion tracking control?Is it not possible to arrange for a friction wheel on the base of the mount to be connected with a flexible axle to a knob of sorts so one can use that to track an object rather than pushing the scope? Or is this only workable on equatorial mounts? Where I live (lat 60 deg N) the pole is almost overhead (30 deg off) so the azimuth is not far from the polar movement. For objects like Jupiter the most movement seems to be in azimuth at least when I tried it out a few weeks back.But then again it might be too much trouble for something that is not 100% anyways...
nfredrick2002I used a large all metal lazy susan type bearing on my Dob and I end up having to tighten the center bolt as it moves too freely now
Just a question:
How is the bearing secured to both mount bottom disks? In the images I have seen there are screw holes, but once the bearing is secured to one disk and the other is placed on top, there is no access to the Lazy Susan bearing anymore....So how is it done in practical terms to mount one like in the following link?Lazy Susan 300mm