Composite Balance Shafts assembled by cold welding press-fit parts with Trib-Joints after treatment with a Trib-Tool.

Application Note 14.

The secondary harmonic balancer was first used on an automobile engine in 1911 by Lanchester and improved by Mitubishisi in 1975. It is now widely used on 2 litre and above four cylinder units. The original Lanchester shafts would probably have been made from individual components by brazing eccentric weights onto bars. Today balance shafts are predominantly solid cast iron because of cost and as a result are cheap but heavy. Because of the need to locate the balance weights midway along the engine block their drive shafts are disproportionally long and heavy.

By making the drive shafts hollow typically 0.4 kilo/shaft can be saved - making a total of 0.8/kilo engine. Because of the inherent out of balance of these shafts the grinding of the bearing lands on cast iron balance shafts can be troublesome.

The picture above shows the individual components and an experimental design assembled with Trib-Joint press fit cold welds. First the weights are precision fine-blanked to provide good bore geometry and create a snug press fit to the shaft. The shaft is a standard welded drawn over mandrel tube providing consistent wall thickness with minimal eccentricity. The bearing journals are off the shelf pre-ground and a press fit onto the shaft. The drive spigot is machined to be a good press fit so it cold welds into the end of the shaft as it is inserted after treating with a Trib-Tool. The shaft is 20mm diameter. with a nominal wall of 3mm. There are several options for assembly and these influence pre-sizing. For instance if the tube is mechanically expanded during assembly it can be made to take up a considerable variation in tolerance variables. Typically the bearing journals are pressed into place dry. The shaft is treated with the Trib-Tool just ahead of their final position so they cold weld for the last 5mm of their travel. The weights are then individually pressed on and taken to within 5mm of their final position. Treatment is preferably applied to the shaft as each weight is pushed home. Finally the drive spigot is pressed in and the last 10mm of its travel is treated with a Trib-Tool.

The shaft is chosen to have a torsional strength of about 200N.m. and the drive spigot and weights joints are sized to match this. The entire assembly is made with mild steel (except for the journals) and it is assembled to finished size - with no finish machining needed. The assembly can be readily automated.

for more information email: info8@tribtech.com

The TribTech name derives from "tribos" - Greek for 'rubbing'. 'TribTech' is a trade name used by Ball Burnishing Machine Tools Ltd. of 12 Brookmans Av. Hatfield, Herts. AL9 7QJ. United Kingdom;  Company Reg. No. 1408807, VAT Reg. No. 421 6210 04; a knowledge based company that develops, patents and licenses technology. All rights reserved by Ball Burnishing Machine Tools Ltd. Last modified: May 01, 2008 copyright © 1999/2008. The information and data provided herein should be considered generally representative for the tools and technologies described. In all cases users should carefully evaluate the tools and technologies to determine their suitability for a particular purpose.