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Motorized Small Boring Unit

Small Boring Machines

Make the grade with a Robbins Motorized SBU

Perform extraordinary bores with Robbins Motorized Small Boring Units (SBU-Ms). Bore longer tunnels, through harder rock, perfectly on line and grade when using one on your next project. You know how well the Robbins SBU works; now experience the same results with specialized boring machines.

When line and grade are critical, such as on gravity sewer installations and/or limited easement access, you need a machine with continuous steering. For longer bores (over 500 ft/ 150 m) in hard rock, you don’t want to be limited by your auger or consumables such as cutters. If you need to bore through mixed ground or hard rock with a casing diameter between 48″ and 78″ (1.2 to 2.0 m), the Robbins SBU-M could be the right solution for you. Robbins offers customized SBU-Ms to fit your job perfectly and allow you to bore more challenging drives.

What is an SBU-M?

The SBU-M is a motorized, manned-entry rock boring attachment meant to bore in a variety of rock types with Unconfined Compressive Strengths (UCS) of 4,000 psi to over 25,000 psi (25 to over 175 MPa). It is made for use with any Auger Boring Machine from 48″ to 78″ (1.2 to 2.08 m) or any standard pipe jacking unit.

The SBU-M is made for longer bores, and maintains line and grade for the length of the bore. The SBU-M is similar to the Robbins Small Boring Unit (SBU). If you are not familiar with the SBU, please familiarize yourself with that machine by visiting the Robbins Small Boring Unit page.

The SBU-M is a similar concept to the SBU-A with the following additional features:

  • In-shield drive motor
  • Pipe jacking option
  • Manned-entry for laser-guided steering
  • Access to the cutterhead while boring to allow for changing and repositioning cutters


The SBU-M uses a hydraulic or electric motor to generate torque. The auger boring machine still applies thrust to the steel casing.

For spoil removal, an invert auger enclosed in steel casing runs beneath the bearing housing and drive motor assemblies. To seal off the cutterhead and protect the motor, a steel bulkhead is added to direct the rock chips into the invert auger.


The SBU-M stays on target because it can be steered continuously, either from the bore pit or from an operator’s console within the machine’s shield. The SBU, in contrast, has only limited steering after the first 20 – 40 ft (6 – 12 m) of the bore. Because of the SBU-M’s steering configuration, you can instantaneously check line and grade at any point during the bore.

Steering is accomplished in the SBU-M via an articulating front shield. A laser is set up in the back of the bore pit and projected onto a target inside the machine. The operator can then control the machine using stabilizer pads and hydraulic articulation cylinders that pivot the machine on an articulation joint.


The SBU-M can bore through hard, abrasive rock more effectively since the cutters can be changed at any point during the drive, eliminating the problem of removing the casing completely for cutter changes on longer tunnels. Another advantage is cutter positional interchangeability. As cutters typically wear unevenly along the cutterhead, each Robbins cutter can be moved to optimize disc wear. Cutters can be rebuilt with a hydraulic press and common shop tools, reducing overall cutter costs.

Like the SBU-A, the SBU-M can be used in both mixed ground and hard rock conditions. The hard rock cutterhead features single disc cutters. The mixed ground cutterhead is mounted with a combination of either single disc cutters or two-row tungsten carbide insert cutters with carbide teeth depending on the ground conditions.