Flywheels & Flex Plates

BTI engineers and manufactures a complete line of manual, semi-automatic, and fully automatic precision measurement and testing equipment for all flywheels, flex plates, and related assemblies and components

A flywheel looks like the simplest rotor in an engine and it carries one of the most misunderstood specifications. For an externally balanced engine, the flywheel is not supposed to be balanced to zero. It is supposed to carry a specified unbalance, because it is part of the crankshaft’s balance system rather than an independent rotor.

Balance Technology Inc. builds manual, semi-automatic and fully automatic balancing and measurement equipment for flywheels, flex plates, dual-mass flywheels and the ring gears and clutch surfaces that go with them.

Internally or Externally Balanced

On an internally balanced engine the crankshaft carries all of its own correction, and the flywheel is balanced to as near zero as the tolerance allows. On an externally balanced engine the crankshaft is deliberately left with a residual, and the flywheel and damper carry offsetting mass by design.

That distinction decides the whole process. A flywheel for an externally balanced application measured against a zero target will be rejected as grossly unbalanced when it is exactly correct. The target is a specified amount at a specified angle relative to a datum feature, and the datum matters as much as the amount.

It also means these parts cannot be interchanged between engine families that look otherwise identical, and a production line handling both needs the part identification to drive the acceptance criteria rather than relying on operator memory.

One Plane, Usually

A flywheel is a disc: large in diameter, short along its axis. There is rarely enough axial separation for a meaningful couple to develop, so single-plane correction is normally sufficient. Thick flywheels and dual-mass assemblies are the exceptions and want checking rather than assuming.

Correction Without Touching the Friction Surface

The clutch face is a functional friction surface with a flatness and finish specification, and correction cannot come from it. Drilling or milling removes material from the back face, the outer rim or designed bosses, within limits set by section thickness and by the stresses a flywheel sees at speed.

Flex plates change the problem completely. They are thin stamped steel, flexible by design, and there is no material to remove. Correction is by weights attached at designed locations, and the plate’s own flatness and distortion become part of the measurement: a flex plate that is not flat will not give a repeatable reading on any machine.

Ring Gears and Runout

The ring gear is normally an interference fit, and it can sit slightly eccentric to the flywheel axis. That produces runout and, because the ring gear is a significant mass at a large radius, it produces unbalance as well. A flywheel balanced before the ring gear is fitted is not necessarily balanced after.

Ring gear runout also has consequences independent of balance, starter engagement and tooth wear, so it is worth measuring as its own characteristic rather than inferring it from a balance reading.

Dual-Mass Flywheels Are Two Rotors

A dual-mass flywheel couples a primary and a secondary inertia through an arc spring system. Each mass has its own balance condition, the coupling is compliant, and the assembly’s dynamic behavior is not that of a rigid disc. Measuring one and assuming the other does not work, and the fixturing has to control the relative position of the two masses during measurement or the reading will not repeat.

Setting the Tolerance

Where the target is zero, flywheel tolerances follow ISO 21940-11 from a balance quality grade, the rotor mass and the service speed. BTI publishes a free ISO 21940-11 balance tolerance calculator with the method behind it: grade selection, the permissible unbalance formula, plane allocation, and a worked example.

Where the target is a specified non-zero unbalance, the standard governs the tolerance band around that target, not the target itself. The target comes from the engine designer.

What Else Gets Measured

Flywheel lines commonly combine balancing with:

• dimensional gauging of the clutch face, flatness and parallelism
• ring gear runout and concentricity
• thickness
• hardness
• crack detection on castings

Surface finish on the friction face is frequently a specified characteristic in its own right.

Equipment Configurations

Layouts range from single manual stations for remanufacturing and low-volume work through to fully automatic cells with drill correction, part identification and data collection, and vary to accommodate any production rate. For platform selection across the full range, see BTI balancing machines.

Related applications include crankshafts, clutches and components and transmission components.

Frequently asked questions