5 Roots Blowers, Synchronising Gears and Rotor LobesThe Mercedes Benz W165, 1.5 Litre, V8 engine was supercharged by an engine driven, two stage Roots Blower or compressor.
Both blowers, the larger first stage low pressure compressor, and the shorter second stage high pressure compressor both run at the same speed, the two pairs of rotor lobes are the same diameter; but differ in overall length. Both are straight two lobe rotors without twist, which would normally create significant pressure pulses. The Mercedes Benz engineers cleverly designed asymmetry into the positioning of the inlet and outlet ports of each compressor to broaden, or spread out, the flow of pulsations of compressed air. The pulses of air from the upper and lower paths of each compressor arrive at the outlet port at slightly different times.
The compressor lobes are essentially a pair of two tooth gears. A pair of gears with only two teeth will not drive each other continuously, so an additional pair of external gears are required to keep the two compressor lobes synchronised. Unlike the synchronising gears, the two compressor lobes must not be allowed to touch anywhere when rotating, otherwise they will quickly seize. Excessive clearance would lead to unacceptable compression losses. The synchronising gears therefore need to be very precisely made to minimise the clearance between the two lobes and prevent contact at all times.
The first job was to make the gear blanks and shafts complete with keyways. Note the special tube nuts.
The overall machining of the gear blanks was completed before the teeth were cut.
Gear cutting underway. The tooth form is 1.0 module. I used involute cutters from China, they seem adequate for the job.
I cut enough synchronising gears for my two engines plus a spare set.
The full size compressor rotor lobes were made by thin wall casting. I decided to machine my lobes from solid. there were made in sections to allow the centres to be hollowed out to reduce weight and help heat dissipation. The individual sections are to be bonded and pinned to the shafts.
The gears and shafts are hand selected for minimum backlash. I found it easier to check the gear meshing with the gears at the wrong end on the blower casing. It was almost impossible to do it inside the gear casing at the back of the blowers. Once the gears and shafts were selected they remain as a matched set forever.
Three lobe sections are bonded to the High Pressure compressor shafts
Five lobe sections are bonded together to form the longer Low Pressure compressor.
The two lobes and their synchronising gears are trial fitted. The lobes are slightly oversize at this stage
The slow process of hand fitting the lobes to the compressor housing begins.
First the rotor lobes are made to fit the housings, with a few thou. clearance, by careful hand scraping and fine 'wet-n-dry' abrasive paper.
Then the clearance between the individual rotor lobes is hand worked so that the two lobes do not contact each other but with the minimum of clearance.
This seems to be a never ending process. Even the minimal backlash in the synchronising gears allows contact between the rotor flanks. Assemble, test, remove contact patch, reassemble, test, remove contact path........repeat........repeat.
And all in the name of model engine making.
The next parts to be made are the external manifolds which join the two compressor housing. That will be all precision machining and a welcome rest from hand fitting.
