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Tolerance Was The Root Of The Matter

In July, 2006 we took on the repair and refurbishing of a large Roots positive displacement blower. The unit is one of six used by a water treatment plant in the New York area. These are low pressure, high volume units that are used to aerate the water in the treatment tanks. In addition to oxygen aerating the effluent, microbes are placed into the tanks to scour and consume the sludge. The aeration stimulates the microbes so they eat and procreate at a much higher rate!

Removing the shafts with their gears and lobes attached was a straight forward operation other than the room temperature. The ambient temperature was over 100 degrees accompanied by by a steady deafening hum of the diesel engines and the blowers. An in-house crane was used to lift and move the shafts once the cover and bearing caps had been removed. In order to function high above the floor in 120 degree heat, the crane operator was fitting with a cool suit and limited to 20 minute stints. The installation on a pleasant fall day was a delight compared to the previous visit.

The blowers were installed in 1973 and have performed well. This 30 plus year old blower follows a traditional twin lobe design, but it is a bit larger than most, pushing 21,000 cfm at 327 rpm. The twin shafts are over 12 feet in length, each weighing 8,000 pounds, and the lobes themselves are 6 feet long by 6 feet wide. The drive gears on each shaft are 4 feet in diameter and weigh 1200 lbs each.


A 150 year old design isn't high-tech, but it still required a craftsman's touch and sophisticated approach to ensure the repair was done right.

Service Tech
While all the measurements and tolerances are available from the manufacturer, there is a bit of a "black art" to setting up the gears. To ensure this went smoothly the manufacturer provided an experienced service technician to be on site during the installation.
However, due to the type of blower and its age the company had to ask one of its retired service managers to help out!

Lobes The tolerance between the two lobes is critical. Since there are no gaskets or seals, the closer the lobes are designed to rotate by each other the more efficient the unit is. Since the lobes do not touch there is no need for oil or lubricants. The gears are splash lubricated.

Gear On Shaft The gear is solid and the bore tapered with channels cut into the bore area. The gear is placed on the shaft and it is pressed as far up the shaft as it will go, leaving approximately 1" of travel before it is seated.

Gear On Shaft At this point a hydraulic pump with a capacity in excess of 35,000 psi is attached to the gear hub. The hose from the high pressure hydraulic pump is attached to a fitting on the hub of the gear. The hydraulic pressure inside the gear actually expands the bore allowing it to move into its final position on the shaft.

Gear On Shaft At first glance it appeared that this might be a heat fit since there were no external clues to indicate how the gear was attached to the shaft. But where heat may work for bearings, etc. applying enough heat to expand the bore on these gears would have ruined the gears themselves. The gears were designed to be installed and removed using hydraulics.
  The blower consists of two assemblies, each with the cast lobes, shaft and a drive gear. The 33 year old drive gears were replaced although there was no readily apparent damage such as broken teeth, etc. but wear was evident by the noise generated by the lobes and the gears. Since these gears are critical to the timing of the blower assembly, brand new gears were installed.

How it works
While the lobes do the work, the set of drive gears is the heart of the blower. The power to move the blower comes from a 1000 hp diesel/flex fuel engine which drives on shaft/gear and that in turn drive the second shaft and its lobe. The result is a coordinated rotation where the first lobe collects incoming air, pass the air to the second lobe and forces it out of the blower. The unit does not compress the air. That occurs when the air passed into a confined conduit.

The lobes rotate by each other with no seals or gaskets, so the tolerances between them are extremely tight (from .0012 to .0040 inch depending on the location). his critical gap between the lobes is adjusted by the alignment of the gear set. The drive gear is permanently set onto the shaft in our shop and the driven gear is partially set on the shaft allowing for final adjustment once in place in the blower casing.

  Basic Design
The basic design of the Roots blower has not changed for more than 150 years. Yet in specific applications it is still a very efficient air handler.

Since the actual installation and removal of the gears is complex and time consuming, it is important to understand all the subtleties involved. The adjustment has to take into account both the gear and lobe tolerances. Although there is a fairly large range in the operating temperature, an unanticipated rise in the temperatiure of the lobes and gears will result in the tolerances being reduced and damage to the blower.

The "black art" aspect of the work comes into play when the gear moves into its final position. The gear moves. The trick is to anticipate how much and in what direction so that when the gear locks into place, it's the correct position.

Going with the flow...
As described above, setting the right tolerances is one thing, setting the gear in place is another. It is not a simple matter of backing off a bolt and resetting the gear.

The gear and the shaft are tapered and that allows the gear to slide up the shaft. Unaided, the gear will reach approximately 1" from its final position on the shaft before the inside bore of the gear matches the shaft diameter.

Although the gear is a solid machined unit the bore on the gear can be expanded to reach its designated position on the shaft. This is done through hydraulics. The gear has channels in the bore and up into the collar of the gear. A high pressure line and pump are attached to the opening in the face of the gear.

A high pressure hydraulic pump is needed to supply enough pressure to expand the gear bore. As the correct pressure (35,000 psi) is reached the gear will move and finally "pop" the gear into its final position.

There are times when the "personality" of the gear is not inclined to cooperate so easily and requires some additional finesse or pressure to get the gear to seat. This is where experience comes in on how to apply that pressure so it is balanced and doesn't wedge the gear on the shaft.

Right the first time
The time and effort taken to align and position the gear is well worth it to avoid having to back everything off and go through the procedure again to get it right.

The shop work and the onsite work were not unusual, but did require attention to details when reinstalling gears, o-rings and bearings. Due to the tight tolerances at the end of the mechanical line, each element was critical in its own way. There was no "good enough" factor.

The blower, fully encased, is powered by a 1000 hp inline 6 cylinder diesel engine. The engine can be fueled by natural gas or methane in addition to diesel fuel. A "flex fuel" engine long before it was a popular idea.

While the diesel fuel and the natural gas must be purchased, the methane is a by product of the decomposing sludge right at the water treatment plant and is basically free fuel.

  The roots of the low pressure air handler

A Roots blower is a positive displacement machine that uses two or more rotating lobes in a specially shaped cylinder, usually shaped like the figure 8. The lobes intermesh with each other using timing gears and suck air in from one side of the figure 8 to deliver the air on the other side. There is no actual compression ration built into the machine; it is simply an air mover. Compression is caused because the induced air is forced into a closed conduit, thereby the atmospheric air becomes pressurized after it has left the Roots blower.

Roots Where did the Roots compressor come from? The principle of the Roots compressor/blower was discovered by two brothers in the mid 1800's. They were Philander H. and Francis M. Roots, joint owners of a woolen mill in Connersville, Indiana. The original design was executed in wood, but it quickly became evident that humidity and water would swell the lobes and render the blower inoperable.

The positive displacement blower was named after them, and the original Roots Blower Company was formed. However the name is widely used by various manufacturers to describe a generic type of compressor/blower.


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