Grinders (115mm - 10k RPM) (230mm - 6k RPM) - Differences?

[irlforum]

Hi,
I just found out this evening that my 115mm grinder goes up to 10k and my 230mm only 6k?

Why is this and what is the main advantages and disadvantages between 115mm and 230mm? Is it just that the higher wattage equals more torque in 230mm machines?

I'm probably opening up a can of worms but I just want to have a general understanding in my head of what I'm querying!

 

[brightspark]

the larger the wheel the more surface speed. so need to run slower .smaller the wheel more surface speed needed :so run faster

 

[Hitch]

The periphial speed of the disc will be similar, a lot of discs will have a max rpm, as well as a max speed, measure in metres/second. Iirc most abrasives are rated around 80m/s.

Beceause a fixed point on a 9" disc has to travel further than the fixed point on a 5" per revolution, on the larger machines need to run slower.

We can work out the speeds if you want to...

 

[astro_al]

Its probably a limitation on the tensile strength of a disc. The greater the radius, the more it 'wants' to fly apart as it spins at a given rate. So to keep the disc from exploding on you, the larger disc needs to spin at a lower rate.

Al.

 

[oilcooled :)]

i could be wrong in saying this but if you put a 9inch disk onto a 4" grinder, which i done before "VERY BAD IDEA" it will spin at the same speed as on the 9" only that the gearbox and bearing and motor cant handel the strain. im trying to say i think spindle speed is the same?

 

[MarkMG]

230mm disc, edge speed new = 4,332 m/min
115mm disc, edge speed new = 3,611 m/min

 

[rtbcomp]

i could be wrong in saying this but if you put a 9inch disk onto a 4" grinder, which i done before "VERY BAD IDEA" it will spin at the same speed as on the 9" only that the gearbox and bearing and motor cant handel the strain. im trying to say i think spindle speed is the same?

The disk will spin at the RPM of the grinder, ie too fast for the 9" disk, presenting the dangers illustrated above.

 

[brightspark]

the spindle speed on the 4inch will be faster than on the 9 inch giving the surface speed at the cutting edge of disk both the same speed

 

[oilcooled :)]

but the rpm of the grinder is rated with a the correct disk fitted. lets 10,500 for a 4.5 and 6600 for a 9inch. so if you have a 9" disk half worn to 4.5", its travelling at 13200, im nearly sure thats right

 

[Hitch]

A half(ish) worn 9" disc will be running about 40m/s. (at 6600rpm)
A new 9" disc will be running about 79m/s (at 6600rpm)
A new 4.5" disc will run around 66m/s at (11000rpm)

The closer to the centre, the slower the speed,

(0.115 x 3.142) x 6600
___________________
60
=39.74m/s