Dowel Bar Inserter

Overview

A dowel bar inserter (DBI) is an attachment carried behind a slipform concrete paver that places steel load-transfer dowels into the transverse joints of a concrete pavement while the concrete is still plastic. Dowels are smooth round bars, usually 32 mm diameter and 450-500 mm long, set across each contraction joint at 300 mm centers. They carry shear load from one slab to the next so that traffic crossing a joint does not cause one slab to step down relative to its neighbor. Placing them mechanically as the slab is paved removes the separate step of pinning dowel baskets to the base ahead of the paver, which speeds the operation and removes a basket that can shift under the paving train.

The attachment indexes itself to the joint pattern using the paver travel, lowers a row of vibrating forks that each grip one bar, pushes the bars down to mid-slab depth, withdraws, and then closes the surface with a finishing float. The whole sequence is timed against the paver advance by the Control System so the bars land at the correct longitudinal spacing without stopping the paver.

Carrier Frame and Lift

The Main Carrier Frame is a welded steel frame that spans the full paving width on two Side Support Leg supports. The Transverse Cross Beam is a box section carrying the transverse loads from the insertion heads, and it must stay level so every bar reaches the same depth. The Frame Lift Cylinder pair raises the frame clear of the fresh slab between cycles and lowers it for each insertion. The heads ride across the beam on a Head Traverse Rail supported by Linear Guide Rail carriages, which lets one or two heads index to each dowel position rather than fitting a separate head per bar.

Insertion Heads

Each Dowel Insertion Head does the real work of the machine. The Dowel Insertion Fork grips a single dowel and a Insertion Vibrator Motor shakes the fork at 8000-12000 vibrations per minute. The vibration fluidizes the concrete immediately around the bar so it parts cleanly and then closes back around the bar once the fork withdraws. Without vibration the bar would drag a void behind it and the concrete would not consolidate against the steel. The Insertion Push Cylinder drives the fork and bar down to mid-slab depth, and a Pressure Sensor reads the insertion force so the controller can detect a jam or a missed bar. The vibrator uses an Eccentric Mass spun by a Hydraulic Motor, with the mass shaft running on a pair of Ball Bearing units sized for continuous out-of-balance load.

Magazine and Feed

Bars are stored in the Dowel Magazine, an inclined rack holding 30-60 epoxy-coated dowels that feed down by gravity. A Bar Singulator Gate gate releases exactly one bar per cycle to each head, and the Bar Feed Cylinder transfers the released bar into the fork jaws. The singulator escapement uses a Gate Finger Plate finger and a Coil Spring return so that the rack cannot dump two bars at once. Reloading the magazine is the main interruption to paving, so crews top it up during planned stops rather than letting it run empty mid-pass.

Hydraulic System

All of the actuators draw from the Hydraulic Power System, which is fed from the paver power unit. A Hydraulic Pump supplies 250 bar through a Directional Valve Block of solenoid Solenoid Spool Valve sections, one per circuit. A Piston Accumulator stores energy so the push stroke does not starve the vibrator circuit during the high-flow moment of insertion. A Pressure Relief Valve caps the system at 260 bar to protect the pump and cylinders. The pump rotating group runs on Ball Bearing supports and is sealed against the case with an Oil Seal.

Finishing and Surface Reclose

After the bars are placed, the Finishing Pan Assembly passes over the disturbed strip. The Float Pan Plate is a stainless float that recloses the surface, and a Pan Vibrator consolidates the concrete so no fork marks remain. The pan is suspended on Coil Spring mounts and lowered by a Pan Lift Cylinder. A poorly recovered surface shows as a rough band over each joint, so the finishing pass is set up carefully and inspected on the first joints of a shift.

Sensing and Control

The Sensor Suite gives the controller what it needs to place bars accurately. A Insertion Depth Encoder on each head measures fork travel so the bar lands at mid-slab depth within +/-5 mm. A Travel Position Sensor tracks paver advance and times each insertion to the joint spacing, and a Inclinometer keeps the cross beam level so every bar sits at equal depth across the lane. The PLC Module runs the cycle and the Operator Console in the paver cab lets the operator set spacing and depth or take manual control over a bad joint.

Mounting and Field Use

The Paver Mounting Kit bolts the frame behind the slipform paver through a Mounting Bracket pair, with Pivot Pin hinges that let the frame float over an uneven base. On a typical paving train the inserter follows the paving slab and precedes the texturing and curing equipment. Crews check dowel alignment with a covermeter or a proof core on the first joints, since a bar that is tilted or off depth reduces the load transfer it was placed to provide. The bars used are smooth and epoxy-coated so they can slide as the joint opens and close in cold weather while resisting corrosion in the saw-cut joint.