Two dryers spinning no heat

I had a dryer that the heating element kept going out. We replaced it twice, and then the circuit breaker started tripping on it so that it constantly had to be reset. The dryer would spin but not heat up. When we reset the breaker it would heat up again, until the breaker stopped working as in we would reset it, and it would flip right back off. Threw out the dryer and got another one that we know works, same problem, spins but no heat. Then we replaced the circuit breaker, which is now in the position it should be, but still no heat in the replacement dryer. Does anyone know where the problem could be?

You will have to check the power to the unit.

Try flipping the breaker off/on slowly a couple times. Sometimes you can loose half the line without actually tripping the breaker.
Check the voltage at the plug
L1 to L2 should be 240 volts
L1 to Neutral and L2 to Neutral, both should be 120 volts.
If OK
Unplug the unit and check the wires at the terminal strip in the machine to make sure none are loose or burned out
If OK
Check the power at the terminal strip.
Do this with the heater off and on.
Be careful as 240 volts is lethal !!!