Vibrations Isolation Efficiency Targets for Power-train Mounts

While cascading Vehicle-level NVH performance targets to those of its subsystems and components, it is not an easy task to sign-off these individual ones for engineering sign-off.

For example, a Tier-1/2 supplier of power-train (PT) mounting would be awaiting, from its OEM, a green signal to their design of rubber mounts and associated structural supporting brackets before the mass-production starts.

Now, we have a set of DVP (design validation plan) targets purely from NVH point of view; the heart of them is the Vibration Isolation Efficiency given by all PT-mounts along 3 VCS axes. It is calculated by measuring the vibrations across them: passive side (body side) versus active side (engine/ motor side), for a particular frequency, at both idling and in full running of the vehicle whose NVH need be signed off. More than 90% isolation level is expected for the least Tactile vibrations and the minimum structure-borne noise inside the cabin.

Now, there is a belief that the rubber-mount design alone determines the above Iso. Eff. If the accelerometer below it shows 95 % less vibrations than from the upper side of PT, then we shall consider it to be the best designed mounting in terms of its rubber-shore hardness or tri-axial stiffness & damping and the mount- location w.r.t. Toque Roll axis of PT.

This is right to some extent in the sense that, softer the mounts, more is separation of PT -rigid body modes' natural frequency from the lowest excitation frequency of either engine-firing or motor-electromagnetic forces and hence, as per an above chart and hence, higher is the Iso. Eff.

This is, however, true only under 2 conditions:

(i) a supporting body-structure from passive side has 10 times dynamic stiffness as that of the rubber mount in the entire frequency range

[for EVs, the motor-train whine is dominant up to 1500 Hz and no structure can avoid the resonance till this frequency; even for ICE powered cars, too, we always find drops in the body dynamic stiffness at certain frequencies till 400 Hz, ].

(ii) there are no other transfer paths from PT to the vehicle body other than 3 or 4 Mounts

[for ICE cars, there are exhaust hangers, intake manifolds, fuel lines, etc, which transfer vibrations from PT to the Body apart from radiator hoses, drive-shafts & suspension pivots which are common in EVs, too].

In either of the above cases, the mount Iso. Eff. will be away from the target > 90 % ( or sometimes can go below 0% ) and then there will be heavy pressures from the OEM Management to redesign the rubber-mounts -- its stiffness or location or even costly hydra-mounts will be suggested! This should be avoided by expert NVH Managers with systematic Transfer Path Analysis.

In modern EVs, often a cradle or a sub-frame (on which the electric motor is mounted) is itself isolated with rubber bushes on the car-body. This is to avoid effect of its resonance on the structure-borne noise inside the cabin from 200 to 1500 Hz. For such a system, the vibration Iso. Eff. will be the resultant of both the primary and secondary isolators' Eff.

For transient events like key-on-key-off, tip-in-tip-out, launch judder, ride over pot-holes, sudden gear-changes, cornering manoeuvres  etc. where excitation frequency ranges from 0.1 to 10 Hz… the mounts are supposed to allow small PT-movements with their internal stoppers coming into action. Here, the isolation eff. calculations have no relevance.

For large Trucks or Buses, even if the Mounts are not well isolating the vibrations from PT to the body or chassis, due to very low level of the passive side vibrations thanks to heavy sprung mass of the vehicle, the Iso Eff. is calculated to be high! A reverse is true for lightweight 2-wheelers!

Hence, the DVP target of the PT-mounts Iso. Eff. need be set carefully during sign-off! The same is true for other applications such as Body mounts or suspension pivot bushes or A/C compressor / Radiator Fan assembly mounts or exhaust system hangers or Battery isolators or valve- Tappet cover gasket, or timing pulley cover isolation.