Roots & Twin Screw Supercharger Assembly Technical Overview...

GEARS & OIL

The gears within these Eaton units and any other Twin Rotor Supercharger are generally of Spur gear type but there are also designs which utilise Helical gears. Both gear types have their characteristics and benefits, but whichever method of gear is used they both have a significant purpose of vital importance.

With an Eaton, any other type of Roots Supercharger or a Twin Screw design the rotors are designed to rotate and mesh together in order to move the air through the Supercharger unit. The tolerances between these Rotors is small and the timing/phasing of one rotor to another is imperative.

This timing task is of course undertaken by the gears in the housing usually at the front of the Supercharger. These gears are interference fitted to the Supercharger Rotor shafts with a very tight fit to prevent the slippage and false movement of them as the unit does its work. Helical gears usually call for a different method of attachment.  The correct fitment and timing of any type of these timing gears and ultimately rotors is not something that should be attempted without the correct tooling and knowledge. Poor attempts to rephase gears and rotors will potentially warp gear bearing plates allowing rotor to case and/or rotor to rotor contact. Although in most cases this transmission is of just Two gears, step up ratios with a sequence gear train can also be found in some Superchargers. Whichever the gear layout situation these gears rotate at significant velocities and in larger models can transmit significant amounts of torque (moving and compressing air is hard work at significant pressure ratios)

It is extremely important that the Oil is of the correct viscosity and specification with certain properties that can resist heat cycles, oxidation, foaming, ability to carry load, maintain film thickness and provide cooling to the gear teeth. Challenging, considering that the volume of Oil in these transmissions is usually not large,100-200Ml. Oil level is equally important, enough so that when the Supercharger is operating on a level Plane the gear teeth dip in to the Oil reservoir enabling gear tooth cooling and splash lubrication to the bearings. Under filling the gear case will obviously lead to accelerated heat and wear in both gears and bearings. Over filling gives rise to churning, movement of the excess Oil by the gears and if extremely over filled, the bearings also. This can increase the power consumption by the Supercharger, the churning with reduced case air volume can build up significant heat, over heating the Oil and build pressure in the case leading to leaks.

BEARINGS, SEALS & GREASE

Eaton Superchargers, M Series and TVS and Twinscrew commonly have Ball Bearing support to the timing gears and rotor at the outlet end of the unit with Needle roller bearing support to the rear intake end. There are some instances where power input is by a Jack Shaft and the drive is Via the rear intake end of the Supercharger but the bearing arrangement remains the same.

The Ball bearings in the gear transmission case are splash lubricated with the same Oil that lubricates and cools the gears, although some do have provision for an Oil feed and drain line. With the Sump reservoir type, the correct Oil type, level and health of the Oil, gives these bearings  a good chance of providing many hours of service. But this is challenged by poor attention to the state of Oil in the transmission case and substantial Supercharger over speeding which could take the bearings beyond their RPM/Load limit which degrades their service life.

The front Transmission Bearings are held in place with an Interference fit to their circumference within the Cast Aluminium housing. Further to this with the case of Eaton, the Aluminium parent material is Spun over the bearing to prevent any unintended movement or play. The fact that the original factory takes this approach makes removal of these Gear/Rotor Plate bearings a considerable task. Any attempt to short cut and merely press out the bearings will force the arrangement against something it was designed to resist, and will lead to significant fracture and damage to the Aluminium case.

The bearings can be removed safely and effectively but it is not without a large amount of work. It is for this reason amongst other reasons that many consider the Rotor Packs not able to be rebuilt since the correct processes need to be employed to remove the bearings and allow access to replace the bearings and shaft seals followed by retiming/phase of the rotors and gears. This amounts to time and costs that often make it not viable to attempt Rotor Pack rebuilds.  There are instances and examples whereby some have and do just press out the bearings, but… this will remove vital parent case material and compromise the amount of Aluminium that is locating, supporting, and holding the bearings. Other Roots and Twinscrew manufacturers employ different methods of retaining the bearings in the case.

The OEM Rotor Pack seals can and do fail in time, due mostly to the abrasion friction from the high rotational velocities of the shaft imparted upon the seal Lips. This can then allow charge air boost into the transmission gear case and also allow loss of Oil. The OEM Eaton seals are of Double Lip design and are fragile in their construction and can be extremely fussy and problematic if rebuilding the Rotor Pack, and if fitted incorrectly render them useless.

The Eaton rear Main case bearings in most instances are of the Needle roller type, with either a Machined ground case or of the pressed outer case variant. Other types of Supercharger can  utilise sealed caged ball bearings. In all of these examples the bearings for the most part are integrally sealed and have a small quantity of lubricating grease ‘for life’ in the bearing. The grease for these bearings must provide lubrication under often extreme conditions, high temperatures, high RPM speeds and with minimal quantity of product. It is a high performance grease that was purposely designed to operate with high speed bearings resisting corrosion, oxidation and Oil separation from the thickener which then degrades its lubrication qualities. The Needle bearings are often of factory fitted integral seal type, seals which are often modest in their performance. This over time can lead to grease loss and results in the Needle rollers running on the shaft bearing surface without lubrication, in turn this can wear the shaft and cause excessive play between the shaft and bearing leading to increased rotor movement and potential rotor contact with the case/and or each other.

CASE & ROTORS

The cases are generally of Cast Aluminium Machined construction although some Supercharger types are of extruded or Billet construction. The cases rarely give cause for problems unless previous attempts at repair have compromised bearing bores, location dowels, sealing surfaces, or any poor attempts of ‘porting’ have ruined port timing/area, or there is excessive wear to the rotor case bores. Wear/damage which can be encountered is obviously larger objects or poorly filtered air ingested into the unit in which fine particles act as grinding media between the rotor and case. Other considerations are that the Roots variant of Superchargers, are well documented to giving excellent boost characteristics at from low Rpm with modest Pressure ratios, however, it is equally documented that the Roots type when asked to deliver higher pressure ratios at high Supercharger RPM (especially the M series or any other low Helix) will be operating outside of its peak Isentropic efficiency. What this means is that for every Pound Mass (density) of air that the Supercharger is ingesting, a lower efficiency is causing a higher power consumption drawn by the unit and giving a higher increase of temperature to the Supercharger charge air delivery. This reduction in Isentropic efficiency causes heating that produces very hot air which has an ongoing plethora of effects, but not least is felt by the supercharger causing thermal expansion of rotor components and can therefore potentially lead to case contact.

This problem is not just the preserve of the M series, TVS Superchargers are also of the Roots type and have peak operating efficiencies and if operated significantly beyond these they also will suffer the same issues. But due to the numerous design features of the TVS, not least port area/timing and rotor profiles/helix, they are better suited to higher pressure ratios and Rpm. Equally Centrifugal compressors and the Twin screw designs, Sprintex, Lysholm,Kenne Bell, Whipple etc all have been designed to operate within a certain air flow region at certain pressure ratios with optimum efficiencies. Making a unit operate at elevated RPM outside of its peak efficiency will move air yes, but that air is often Superheated, the density ratio drops right off defeating the objective and will need addressing to prevent potential Engine and Supercharger problems.

The rotors themselves are again of Aluminium, they are supported on Steel shafts with a large amount of interference fit. Rotors have a varying number of Lobes, Helix, Profile and length, all of which can change the characteristics of the Supercharger. Manufacturers will spend huge sums to find the optimum rotor configuration to deliver the intended air delivery performance.

Unless subjected to some form of abuse the rotors will continue to undertake the task for which they were designed, to move and in some instances pressurise the air. But an area that can be compromised is rotor coatings, especially with age, heat cycles and contact friction. Various manufacturers use a number of different methods;  Anodising (not technically a coating) Epoxy, Polymer, Conformal Abrade or even naked Aluminium with Polymer seal strips. All coatings will conform to some degree as the rotors interact, some just more successfully than others.

Some Manufacturers pay extremely close attention to the machining process and have tight tolerances of the rotors and therefore consider coatings unnecessary. Others however employ coatings which are only a few Microns thick to reduce the running clearance margin between rotor to rotor and rotor to case. This reduction in margin helps to reduce air leakage paths through and around the rotors, leak paths which in turn reduce efficiencies of the Supercharger. All coatings go to serve the same purpose and each manufacturer will have reasons for their particular approach, Eaton are currently utilising the conformal Abrade type coating which allows tighter internal  machined clearances since any contact to this type of coating will see the coating yield intentionally.