IMPROVING THE LXD75 MOUNT

This investigation came about after I stalled the telescope mount one night .I know about this standard fault and had not corrected for it so it was totally my own fault that it happened.For some years I have had a vixen great polaris type of mount and this has a bad design fault in the mechanical construction.What happens is that in certain positions the drive motor covers clash with the body of the mount and can stall the motors causing possible wheel and worm damage.The meade LXD75 mount looks on inspection a good copy of this type of design but using DC motors controlled in the standard Meade way not stepping motors in the form that vixen do for their simpler cheaper mounts.Meade also have made an attempt in the software controlling the mount to keep the mount normally out of this perilous state as well as the usual attempt to stop cable wrap problems.In reality the design fault has been fixed in the vixen sphinx design that sorts this problem out with a different motor head design and a single motor cable, so it is nothing new and well known about.I had also noticed that the lxd75 mount was also stiff at one point in rotation of the RA axis causing minor tracking problems at that point on the shaft.As the LXD75 mount is built like motor cars before world war two, you can take it apart easily with the simplest of tools and mechanical knowledge.Those are three hex head allen keys in practice.If you unlock the grub screw in the RA shaft end holding on the metal screw on locking collar then you can remove the entire RA shaft assembly.The reason I did this was that I could not find anything wrong with the worm and wheel other than the lack of grease on it at the time.The adjusters for setting it up are quite simple using a plate and hex head bolts for adjustment of the worm into the wheel.A brass nut sets up the worm slop in its mounting head.After examination I found that the RA shaft assembly was dragging at one point in its rotation on the outer casing.Measuring the casing I found that it was out of circular true by five thousandths of an inch.After squeezing the casing in a vice I managed to spring the casing back into true.The surface that was dragging was the surface used by the brake clutch.The lever on the side of the head rotates a threaded shaft that pushes a cork plug onto this surface inside the mount head.The dragging stopped I then cleaned both gear assemblies of grease and re-lubricated them with Rocol molyslip open gear grease.I have found this grease to work best on all of the drives I have applied it to.It copes with the enormous forces that squeeze other types of grease off the working gear surfaces and the molybdenum disulphide super lubricant in it permanently coats the gear surfaces and improves gear working performance at all temperatures.As the motor assemblies were off the mount I then made sure the clutches were loose.I then connected an ordinary battery powered hand drill to the worm drive shafts after removing the drive gears and ran the drill to turn the shafts for several minutes for both gearboxes.At the same time I applied more grease to the gears making sure everything ran properly with no labouring of the drill motor.This also allows you to set up the worm to wheel tolerances so that they mate together properly.All you have to do is to set the torque clutch on the drill to minimum and use the trigger control on the drill to control the speed.Most of these drills only run at a max of 300 rpm and it is advisable to start off slow then speed up slowly after that to max revs.Never use a mains powered drill as it is too fast and too powerful.Working the worm and wheel at speed is a form of running in and polishing the two surfaces of the gears so they run with less friction.You simply refit the drive gear periodically and turn the shaft by hand.If everything is free and loose then you will feel it in the rotation of the gear.After complete re-assembly I attached the scope and balancing weights.Everything now was so free and easy running that balancing the scope assembly exactly at start of set up became very easy.Listening to the drive motors they had now settled down to an even note when moving that did not have any speed variabilities depending on worm position and shaft position.The current consumption from the power supply had dropped slightly when tracking.After drift set up the mount worked better in tracking performance without any guiding corrections.

Fixing mount jamming problems

Looking at the lxd75 mount design an improvement would be such that if you could extend the length of the driven arms for RA and declination then the motor bodies would have less chance of fouling the casing.In practice it was not practical to extend the declination casing but the RA casing could be extended using a simple aluminium tube and longer 6mm cap head bolts.For this I turned one using the lathe with the correct outside diameter and with a six millimeter wall thickness, a practical length being forty millimetres.After anodising this looked ok as in the picture below.As both the axis were not extended this has offset the vertical loading away from the center line of the mount but this has had no effect on the tracking performance (see picture at end of this page).I am going to look at an electronic solution in the form of monitoring the motor power or monitoring motor pulses over time if I ever get the time to work on it.There is no real total solution to this problem that I can see at present. And as a final must have I made a simple aluminium bar handle that fitted across the tube rings.This made the scope safer to handle especially when wet with dew.The components for this can be found in a DIY store.The brackets were made out of a steel L bracket of the type used for reinforcing wood frames.The bolts were off the shelf plated 6mm bolts.In my case the bar was tapped to 6mm but a length of all thread and two nuts would do the same.At worst case a standard metal furniture or door handle could be adapted.Was it all worth it ..the answer to that is yes on the lubricant performance alone.The extension tube has lowered the chance of fouling but not removed it completely.The handle has stopped me from worrying about dropping the scope.The plus towards everything was the simplicity of the modifications.All of the work efforts only entailed a few hours of work and cheap materials.

Google for the grease.

This is the extension tube on the mount

This is the handle attached to the rings..The connector assembly is for the dew shield heater

This is my lxd75 set up on a steel and concrete pillar

Picture taken of the california nebula....This was three four minute unguided exposures,stacked and contrast and brightness modified plus a slight amount of low pass filtering using paint shop pro to get the light pollution out of it as much as possible.

Everything related to tracking on a scope driven by a worm and wheel depends on three things.

The quality of the machining in the worm and wheel manufacture and the materials used

The lubrication

The set up of the worm wheel itself.

The size of the wheel just gives you the ability to drive a bigger mass of scope and balancing weight assembly.

With this mount as in any other you have to get the worm mounted literally within fractions of thousands of an inch of the wheel.It also has to be stopped from moving backwards and forwards in its mounting bearings.The worm must also be tangential to the wheel and not be sloping in any way.The worm assemblies are mounted on a plate held in place by two hex head bolts with a third for adjustment.With the motor drive assemblies removed.You set the outer pair finger tight at first with the third center bolt removed until the worm is on the wheel.Adjust as necessary until the worm looks square on.The worm should turn by hand and be very slack.Using the allen key tighten the outer bolts by a tenth of a turn.Check again for tightness of the worm.repeat until the worm becomes difficult to move.Replace the center bolt and tighten it with the key while trying to rotate the worm with your finger.You will find that the worm becomes slacker. You are now at optimum adjustment position.Tiny modifications of the tightness of all the bolts will set the worm perfectly.

When the worm and wheel is manufactured it is nearly impossible to cut the gears to absolute perfection for all of the gear surfaces , so rotate the worm around the whole wheel and find the best average setting for the whole wheel that does not tighten the worm wheel assembly too much.There is no spring loaded pressure compensator on this mount so this is all you have to set it up.The working in or wearing in process I mention above helps to even out these high and low spots on the cut of the gears on the wheel.The worm bearing travel is adjusted by undoing the big brass nut and experimenting with the tightening of the threaded bearing until the movement is removed.You then re-tighten the brass nut.In the end the target is to achieve effortless motion of the worm and wheel without any angular slop in the mount that the motor can drive with minimum effort without speed changes as the telescope rotates.

Do not be fooled by setting the worm pressure too slack as this can end up creating more telescope slop movement and wear in the end.Experiment and gain experience and use your head and your own judgement.Remember for this price you have the minimum quality of materials and machining for these critical parts.Only a lot more money gets bigger wheels made out of phosphor bronze or titanium etc.

When you refit the motors again the gear surfaces must mate together firmly but not tightly.Lubrication is not necessary on these spur gears.

Never attempt to drive a scope unbalanced,balancing must be perfect before you even switch on after assembly.