HEQ5 Belt Drive Modification
Posted 15 December 2012 - 09:48 PM
By Malcolm Crabbe
Following on from various posts on the astronomy forums, and research on the web, there was a lot of information regarding the conversion of the Synta EQ6 mounts to belt drive to reduce or practically illuminate periodic error. However I couldn’t find anything on the net that detailed a similar modification to the HEQ5 mount, so decided to do some experimentation. This publication documents my findings and provides details of the resources used should anyone else wishes to undertake the conversion.
My thanks to Chris Shillitio from the EQMOD project for providing me with the gear ratios of the HEQ5 and suggesting which ratio would give the best compromise between slewing and driving.As you will be changing the ratio from the factory settings the mount cannot be used with the Synscan controller. You have to EQASCOM and a PC to drive the telescope through an EQDIRECT cable (DIY or commercial options are available on the net)
One of the other factors was that as I was hoping to fit the pulleys internally and in such a way that I could revert back to factory configuration should the modification not work out. This meant that the maximum pulley I could use on the RA / DEC axis worm shafts was 38.3mm in diameter of 60 teeth.
Therefore a 5:1 ratio dictated that the motor pulley had to have 12 teeth, and both of these can be easily obtained in the MXL pitch. Now there can be quite a deal of torque and inertia when the mount is slewing, so I opted for standard ¼” belts. One other thing that was glaringly obvious was that unlike the EQ6 belt modification; these pulleys would need a lot of machining to fit in the casing.
The parts were purchased from two sources and I sourced someone who had access to a lathe and milling machines. This work can be undertaken by any precision metal works, and shouldn’t cost more than £30-£40. I contacted a fellow forum member who had demonstrated his machining skills and would like to offer my thanks to Tony Rigden for agreeing to undertake the machining of the parts for me.
The large 60t pulley needs to have its thickness reduced to 7mm (I made the originals 8mm as I kept a little of the original hub, which isn’t really needed). The stock thickness of the pulley is 18mm, with a tooth section 11mm thick. So you need to remove 4mm from the tooth section and 7mm (all of the hub). You then need to drill and tap a hole through the pulley to accept a suitable grub screw. I had planned on having two holed drilled at 90 degrees to each other in the same way the original gears have two grub screws, but so far a single screw has worked OK.
I used a 2mm screw, but would suggest using a 3mm if possible. The final work that is required is to bore out the 5mm hole in the centre to 9mm so it fits on the shaft of the RA and DEC worms. The final and tricky job is to bore out the 12t pulley from 3mm to 5mm. This needs to be done very carefully as it’s easy to run out and thus induce a wobble when the pulley is fitted. There is also very little material left once the pulley has beenbored out, which can lead tofailure when the belt is undertension. The pulleys should look a little like this.
The final machining task is to remove the stepper motors from the mount and elongate the slots in the motor plates to allow for tension of the belts. Note that this will invalidate the warranty if the mount is new.( When I purchased the mod from Malcolm I did not have elongate the slots to achieve tension and would advise that you see if you can get decent tension before butchering the slots)
Remove the covers off the gearbox and motor control housing, unplug the power to the mount and remove the two motor connections to the board. Using appropriate allen keys remove the large gear on the worm shafts and then un-bolt the motor plates and remove the motors from the mount. Undo the remaining bolts that secure the motors to the motor plates and elongate the slots in the plate. The pinion on the motor can be carefully pried off, or if you get really stuck use a small drill and a hammer to drift the shaft having supported the pinion accordingly.
To remove the idler gear from the motor plate you need to undo the small grub screw at the base of the hole where the spindle goes. You can then use a suitable spanner to remove the spindle,O ring and intermediate gear.
Re-fit the motor to the motor plate and tighten the bolts. Fit the plate back into the mount, but leave the assembly just loose enough to allowfor fitting of the belt.
Place the 12t on the motor shaft and tighten enough to stop it dropping down the motor shaft. Fit the 85t belt to the DEC axis and 82t to the RA, and then carefully release the grub screw on the 12t pulley to you can get the alignment of the belts and pulleys correct and then tighten it as much as you can but being careful not to strip the
Use a flat bladed screwdriver and place the blade between the motorplate and casing and with a twisting motion put tension on the belt and tighten the bolts securing the plate whilst keeping the tension on the belts. Connect up the motors to the control board and power up the mount. Run the motors at full slew rate to ensure the belts remain tight and track OK between the pulleys.
You’ll notice that the 12t pulleys sit proud of the motor shafts, more on that later.
The next thing to do is fabricate a gasket or spacer to raise the gearbox cover so that it clears the gears. My first attempt was to use a mouse mat, which was carefully cut using a sharp craft knife. This has worked so far, but isn’t the ideal and I’m still on the lookout for something that can be cut easily and provides
the same clearance.
You will also need some longer bolts (10mm long) to secure the gearbox cover to the mount.
The final result can be seen here.
You then need to configure EQASCOM to use a custom ratio. This is documented on the EQMOD website or in one of Chris’s excellent tutorials on YouTube.
The custom mount setting settings for the HEQ5 Pro 5:1 ration are:
Worm steps 64x200x5 = 64000
Total Steps = 64000*135 = 8640000
Tracking offset -28
Make sure the "custom Mount enabled checkbox is checked before hitting the big OK button. You then need to go through the normal procedure of parking the scope in EQASCOM and then manually placing the scope in the default home position and locking the clutches. The mount is then set up for use.
In conclusion the project has turned out to be worthwhile, even if just because I’m getting precise go-to’s. It’s not as straight forward as the EQ6 belt mod, requiring more machining,but it does require less stripping down of the mount and replacing belts and future maintenance is simpler than the EQ6 conversion. Only time will tell if the long term benefit of undertaking this modification has really been worth the effort, but so far I’m pleased with the results.
MXL Pitch Rubber Timing Belts
(4580) Width in mm 6.35 Belt Length in mm 166.62 (82t) - £1.49
(4580) Width in mm 6.35 Belt Length in mm 172.72 (85t) - £1.50
From Belting Online http://www.beltingon...-rubber-timing-
2 x Aluminium MXL Pulley, 12T, 3mm Bore (PL012AL) = £6.92
2 x Aluminium MXL Pulley, 60T, 5mm Bore (PL060AL) = £14.26
From Motionco http://www.motionco....timing-pulleys-
Total costs of parts: £24.17 + Vat = £29.00 + postage
Machining costs: £20 - £40 from a machine shop
Total: £49.00 to £69.00
Mod in action.
Posted 15 December 2012 - 10:36 PM
Fantastic article and mod for the Synta EQ series of mounts; if nothing else it takes away the coffee grinder sound that these mounts are known to make from time to time...
Also, looking at the results, your images, I know that your a lot happier with the tracking of the mount with the belt mod.
Posted 16 March 2013 - 02:11 PM
I did my HEQ5PRO last year.
Beltingonline will machine the pulleys for you. 2 x 60t and 2 x 15t machined pulleys came out at £69.58
Belts are cheap as chips
5 x MXL Pitch Rubber Timing Belts (4580) = £11.07 inc vat+insured delivery
(Width 6.35mm, Length 172.72mm)
I went 4:1 ratio. More meat on the 15t pulley.
"If you get that error message - it means what it says"
Posted 17 March 2013 - 09:58 PM
This is certainly on my to do list its been in the back of my mind for a while. Great article by the way very thorough, and listening to your vid your mount is now silky smooth! I take it you drop less subs now?
Posted 26 July 2013 - 12:03 PM
Perhaps adhesive neoprene would be a useful material to pad out the gearbox cover, it's readily found in various sizes and thicknesses on ebay and is cheap as chips.
Great Mod btw, ever thought of approaching First Light Optics and arranging to sell kits?
Posted 27 July 2013 - 07:27 AM
Pre made kits would be an excellent idea, you could even cut out the middle man and sell them your self