Straight dc motors:
These are motors with no gearbox attached. Generally a brushed DC motor speed will run in lots of 1600 due to the way the motors are wound with common dc motor speeds being 1600 rpm, 3200 rpm and 4800 rpm
The use of these is practical if you plan to make your own gearing with belts and pulleys. Generally, the useable rpm is a little high with hardly any available torque. With the use of a belt and pulley system, you can set up your own easy gearing. For example, a 4" pulley on the application to a 1" pulley on the motor is known as a 4:1 ratio, and if you got 1nm at 3200 rpm from the motor, then you can divide the motor rpm by 4" and multiply the torque by the 4" (3200/4 = 800 rpm and 1nm x 4= 4nm). So in the example given, our 3200 rpm would be reduced to 800, and the motor would now be giving us 4 nM of torque.
It also works in the opposite! if we had gone 1:4, rpm would have multiplied by 4 but torque would be reduced by a division of 4!