In most applications there is no or little differential pressure. Where the rotary seal is exposed to pressure however, the sealing lip is pressed against the shaft, thus increasing friction and temperature. In extreme cases the pressure can even cause overturning of the sealing lip.
Over 0.2 bar at higher peripheral speeds or over 0.5 bar at low peripheral speeds back up rings or specially designed rotary shaft seals (TCN TYPE) with a stronger sealing lip and supporting metal insert must be used. 1 bar = 14.5psi