Roll deflection vs. increasing MPH (magnified)

Model (sectioned views) of typical chassis dyno roll
with callouts for areas of balancing concerns.

3D FEA simulation of roll deflection (each top image
is at 60-MPH with the bottom image at 120-MPH)*
*Note, all model displacements are exaggerated
(uniformly multiplied) to aid the visual analysis.

Analysis Comments

Hover mouse pointer over image (above) to toggle highlighting.

Above image shows (expected) minor irregularities (exaggerated to aid viewing) of a typical chassis dyno roll assembly. These include:

Roll shell imbalances (shown as internal bumps) due to mill forming tolerances, welding imperfections, surface scaling, handling damage, and machining tolerances.

Radial run out, however slight, due to machining fixturing tolerances.

Shaft deflection, even if ground shafting is used, occurs due to construction stresses. Even just a few thousands of deflection can be an issue – as we will explain towards the end of this table.

Hover mouse pointer over images (at left) to show roll at rest.

Baseline "Spin" images show a roll with minor shell imperfections (far-left image) and construction run out, along with  some shaft deflection, all creating imbalances. When spun, the resulting unbalanced dynamic forces cause serious stress and vibration.

At higher speeds, the uneven (end-to-end) variations in mass result in a violent rocking coupling (lower image). This can eventually fail the bearings, the roll and frame welds, or even the axle-shaft's ends!

Keep in mind the actual shell deflection represented by the "red" zones is only about 0.001 inch. However, with a roll that weighs over 1,000 pounds, even that much (one-sided) deformation represents hundreds of gram-inches of imbalance.

Notes:
The upper (low-speed) analysis image is at a 60 MPH (97 km/h) surface speed.
The lower (high-speed) analysis image is at a 120 MPH (193 km/h) surface speed.

Static balance weights on the end plates of roll. Note that placement of the weights is ignorant of uneven end-to-end locations of imbalanced masses. Process is analogous to static (bubble) balancing automotive tires - and suffers from the same shortcomings at high speeds.

The two end imbalances get missed, because they cancel one another during a simplistic static balance. Since no significant RPM is applied to the roll during the static procedure, many such dynamic imbalances get missed. Later, when this roll actually spins in service, its dynamically imbalanced ends will cause it to rock violently!

Static Balancing is totally unsuitable for correcting imbalance in relatively lengthy chassis dyno rolls – that are being used at anything beyond very-low speeds. Notice the significant and uneven (rocking) deflections of the left vs. right axle-bearing stubs.

At higher speeds uneven end-to-end variations in imbalance create a much more violent rocking coupling that can eventually fail the bearings or axle ends. The uneven and opposing directions of the end-end deflections are due to the inability for static balancing to provide proper correction for dynamic imbalances. In fact, static balancing sometimes increases dynamic imbalance – once the roll is spinning!

Above image shows typical "dynamic balancing" via the application of specially positioned external counterweights at the ends of the roll assembly. Dynamic balancing is typically done on a balance machine that spins the roll during testing. Sensors pinpoint the required radial location and mass for a pair of weights to offset the dynamic imbalances as they (cumulatively) appear at the end planes of the roll.

External dynamic balance weights on the end plates of roll assembly oppose their cumulative (and respective) end-to-end internal imbalances. This method is analogous to "spin balancing" an automotive tire. Different amounts of correction weighting is placed on each side of the assembly, at specific radial locations, as indicated by the balancing machine.

Note how the left and right axle-bearing stubs now deflect evenly and in the same direction. The rocking forces are mostly gone.

Unfortunately, at higher speeds the roll still deforms. This means that some of the counterweighting correction that was right for lower speed cases, is now inappropriate for the roll's new (and again imbalanced) shape.

Above image is after special internal (multi-plane) dynamic balancing. Notice the new series of counterweights installed just opposite each of the shell's and shaft's general areas of imbalance. Multi-plane balancing is also performed on a balance machine that spins the roll during testing, but computer algorithms are used to break the dynamic counterbalance points into more than just one pair of plane, radius, and radial locations.

External dynamic balance weights on end plates of roll assembly oppose their cumulative (and respective) end-to-end internal imbalances. This method is analogous to replicating every heavy area on the roll's shell with a mirrored counterforce.

Now look at the roll's high speed shape. Even here the symmetry of the counter deformation continues to keep the shell of the roll nicely in balance.

Unfortunately, the shaft is still a serious issue. As you have seen in every high-speed analysis so far, its tiny (at-rest) bow becomes a major deflection under high-speed centrifugal forces. Because the axle is beefy, the mass shift due to the changing deflection makes it impossible for the roll to stay in balance over a wide range of speeds. Even if this roll was dynamically balanced to G1 specifications at 1,000 RPM, it will be out of balance at any other speed. Understand, these forces increase exponentially with RPM, so if it vibrates at 1,500 RPM, it will be jumping off the ground at 3,000 RPM.

Above image shows how the shaft's bow and run out have finally been corrected. This addresses the last major balancing issue for this assembly.

Shaft assembly machined true (as a subassembly) along its entire length. Notice the shaft's hourglass shape – exaggerated to aid viewing and indicated by the two red arrows. In actuality, about 1/16" (radialy) is all that needs to be removed to correct most subassembly's run out (for the shaft, collars, and end plates).

Notice how the left and right axle-bearing stubs no longer deflect. In both the low-speed and high-speed analysis images you can see that the sources of imbalance, and their respective contribution to uneven deformation, are now balanced by our "mirrored" counterweighting measures.

This roll runs smooth throughout its working range. This is the only correct way to balance any chassis dynamometer roll assembly.