thanks for the replies,i just wonder at times if the motor on an 18v tool is really the same as one on a 14v tool and the only difference being the run time due to the extra cells in the 18v,i'm guessing those dc motors would have some tolerance and work within a certain variation of voltage,perhaps someone with a knowledge of electronics/dc motors could say?
As Ken said, they are not the same motors. You are right when you say that motors have some tolerance in the voltage they work with. Every motor is designed to work with a specific optimal
voltage in mind, but, within certain limits, will work with other voltages too. It depends on how the motor is designed. Some will work with a wide variety of voltages, others will not tolerate more than ±0.5v.
Going to a lower voltage is mostly not a problem for the motor in the sense that it can be damaged. It will still work, but at a lower speed and with less force. At some point the motor will simply not work anymore because the voltage is too low to overcome the internal friction of the motor.
Going to a higher voltage is more dangerous and can easily destroy the motor. Heat is the biggest problem. The motor runs at a faster speed which generates more heat and its casing is not designed to dissipate all this extra heat. The internal electric wiring of the motor is also not up to specs for the higher current, also resulting in more heat generation. It is possible the copper wires simply melt.