Hi to everyone. I have build a large CNC with y = 3 meters and x = 1.2 meters. I have a measured a backlash on a 10 mm movement after directional change: y=0.6 mm x=0.45 mm I do not really know where the weak spot in the mechanic is. Komponents are: - 2xNema 23 on y and 1xNeam 23 on x. But not the biggest driver with max 2.2 Amps - GT2-3m Belt - Stepper Motors are mounted on the gantry. Like on the belt driven Workbee. What I have checked: - wheels not to tight (movable by thumb) - setscrew are tight and on the flat spot - on belt tension I'm not sure. How much kg tension should I put on the GT2 3m belt? I think I have about 4 kg roughly estimated. What is a normal, expected value for x and y? Hints where to find the weak spot.
your belts are probably not tight enough. I use a fishing spring balance to put a known 12kg of tension on my belts. due to your machine length, you may need to go higher, I would try 15 and then 20 if still too mcuh backlash. to pull 15 or 20 you will need a helper, I can oinly just do 12 by myself AND hold the machine still AND tighten the belt fastener.
I'm a bit scared of putting too much strain on the Nema 23 ball bearings. How do you measure the belt tension? I've also heard that if belts are over-tightened, the teeth don't mesh as well with the pulley.
I also suspect that your belts aren't tight enough. Gates give a *minimum* tension figure for GT2 belts to mesh properly: For 9mm wide, 5mm pitch GT2 it's 8.4 lb *per span*, so 16.8 lb = 7.6 kg tension on a pulley with 180° wrap. For a 15mm wide belt, the equivalent figure is 15kg (page 140, table 3) From that page: "Because of the high performance capabilities of PowerGrip belts, it is possible to design drives that have significantly greater load than are necessary to carry the actual design load. Consequently, Formula 4 can provide [static tension] values less than are necessary for the belt to operate properly, resulting in poor belt performance and reduced service life. If a more appropriately sized drive cannot be designed, minimum recommended [static tension] values are provided in Table 3 to assure that the PowerGrip belts are tensioned properly when lightly loaded. Always use the greater [tension] value; i.e., from Tst Formula 4 or Table 3." If the load is too much for the motor bearings, then you will need to mount the drive pulley on a shaft with suitable bearings and couple the motor to it so that there is no side loading on the motor.
@Misterg Thanks for the metrics. @David the swarfer I have such scale. I Will try to measure the belt tension.
Are you sure that the span tension has to be doubled on a belt and pinion system? Is the span length about half of the axis on belt and pinion?
I believe so - My understanding is that the static tension referred to in the guide is the tension in a single strand of belt. The force on the pulley will be the resultant of the tension in the strand of belt running to the pulley, and the strand running away from the pulley. For a 180 degree wrap (as is common for CNC) the force on the pulley will be twice the belt tension. If the tension is adjusted by sliding the pulley with a 180 degree wrap, it’s necessary to put a force of 12kg on the pulley/mount to get 6kg belt tension (as an example). If you read something else into the document, you may well be right.
