Cleaning and maintaining storm and sanitary sewer systems is an important task for state, county and municipal public works and industrial maintenance departments. The machines used for this work require extremely powerful pumps and vacuums to clear these vital conduits. This equipment is constantly exposed to contaminants such as water, debris and caustic chemicals – the worst sort of operating environment for seals and bearings.

   Vac-Con, Green Cove Springs, Florida, is a leading supplier of vacuum sewer cleaning vehicles and industrial vacuum trucks to the government and private contractor markets.
   The Vac-Con Combination Machine is a truck-mounted high-pressure water system and a two- or three-stage centrifugal compressor vacuum system designed for sewer and storm basin maintenance. A high-pressure water jet is used to clear any system blockages. The high-power vacuum draws the material into a truck-mounted debris tank via a large diameter pick-up hose that is lowered into the drain.
   
The two-stage centrifugal compressor is driven by the truck engine through a power take-off (PTO).
   A hydrostatic pump and motor are used to power the three-stage centrifugal compressor. Originally, grease-lubricated pillow block bearings were used to support the compressor shaft, but they did not have adequate reliability. Conventional lip seals could not provide the required degree of bearing protection life due to the high amounts of ambient dust and grit, combined with vibration and very high shaft speeds (5000-6000 rpm). The resultant seal failures caused lengthy and expensive machine downtime and shaft refinishing. The objective for improved seals and bearings was extending service life to five years without the need for planned maintenance. To accomplish that, the seal would need to provide much better protection from water and contamination. In addition, a seal was needed that didn’t generate heat, which in turn reduces grease life. The bearing would be required to withstand high shaft speeds and operate in a very harsh environment.
   
SKF engineers recommended that Vac-Con specify a hybrid ceramic bearing for this difficult application. The hybrid bearing has steel rings with rolling elements made of silicon nitride.
   Hybrid bearings are resistant to wear, high temperatures and corrosion, and they have a reduced lubrication requirement. Expected life of a hybrid bearing can be 10 times that of an all-steel bearing. The hybrid SKF bearing was the perfect solution for the bearing requirements of the Combination Machine.
   
To maximize bearing life still further, the previous seals were replaced with Chicago Rawhide Defender Series^® bearing isolators. These two-piece, compound labyrinth seals feature a mazelike internal structure that forms a series of right angles and pressure drops. These collect contamination and eject it from a bottom-facing port. Their dirt exclusion capability greatly exceeds that of conventional seals. The Defender^™ seal parts are permanently assembled for easier installation, and since its rotor and stator sections don’t touch each other – even during break-in – seal wear is minimized and service time can approach the design life of the bearings. Defender isolator seals do not require lubrification, and friction, power loss and heat buildup are practically zero. The two seal components are PTFE, and fluoroelastomer o-rings are used. This combination results in outstanding chemical and weathering resistance. Using SKF hybrid bearings and CR Defender seals has significantly reduced downtime and maintenance costs on the vacuum trucks.
   
“We searched many years for a bearing that would deliver the performance we needed in the hostile environments where our equipment works,” said Thomas Armstrong, engineering manager at Vac-Con.
   “The answer was the SKF hybrid ceramic bearing. Our blowers work at different angles in the field, and to get this kind of performance from a grease-lubricated bearing is fantastic, especially at these high shaft speeds. In addition to providing the best possible contaminant protection, the Defender bearing isolator lets us run at high speeds and enjoy long grease life. Harsh climates, from the deserts of Saudi Arabia to the snowfields of Canada, haven’t affected performance.”

DEFENDER™ Bearing Isolators

Product Advantages

• Maximum performance in an economical bearing protector
• Maximum corrosion resistance
• Low coefficient of friction
• Filled PTFE materials for maximum chemical resistance and application range
• High speed capability up to 5,000 feet per minute / 25 meters per second
• High temperature capability up to 400°F
• Unitized design holds seal together
• Standard shaft diameter range .500” to 6.00” and metric equivalents (contact Chicago Rawhide for other requirements)
• Dynamic run-out (DRO) capabilities to .020” (.51 mm)

General Specifications
Shaft range: .500” to 6.00” basic sizes
(Larger sizes possible on quote basis)
Shaft speed: up to 5,000 fpm (25 m/s)
Temp range: -40°F to 250°F (400°F specials)
-40°C to 121°C (204°C specials)
DRO: .020” TIR (0.51 mm)
Axial limit: ± .015” (0.38 mm)
Pressure allowance: 0 psi

Standard Specifications

• Carbon/graphite filled PTFE std. under 4.00” shaft,
  stainless steel filled to 6.000” and beyond

• .690” OAL with .316” insertion into bore
• Shaft sizes under 1.750” have diametric cross sections between .700”-1.00”
• Over 1.750” have diametric cross sections between .700-1.250”
• Custom Specifications: Contact CR for application assistance and specification details.