Motion system (chip spreader drive and spreading system)

01

Check gates, roll, conveyor and rate control

FHWA's chip seal checklist (2019) lists what to verify on the chip spreader before work: the gates are adjustable and each gate control and setting has been checked; the roll is straight and not warped; the scalping screen is in good condition; calibration across the entire chipper head has been checked for uniformity as specified or by ASTM standard; truck hookup hitches and the aggregate spreader's truck release latch work; the receiving hopper has no holes or large gaps, and its rubber shield is in good condition; the conveyor belt is tight and has a rubber, neoprene or fabric cowling to prevent aggregate loss; and the computer rate control has been checked for a uniform application rate at different speeds. [1]

The checklist also calls for enough haul trucks to keep up with the spreader's application rate, clean truck boxes, and a truck box apron or extension where needed to avoid spillage when loading the spreader. [1]

AASHTO's construction guide specification for emulsified asphalt chip seals (Section 406) calls for a self-propelled mechanical aggregate spreader with computerised spread control that distributes aggregate uniformly to the required width at the design rate. The contractor must provide proof of calibration of the distributor and the spreader. Transverse spread must be uniform across the veil and within ±10% of the average rate, with ASTM D5624 named as one effective procedure. Longitudinal spread must be within ±10% of the design rate. [2]

The commentary describes a first visual check: variations in light passing through the veil of chips show uneven application, and the machine is stopped and the responsible gates adjusted before rerunning. For the rate, it suggests weighing the spreader before and after, or dividing the weight of each supply truck by the area it covered. In its example, three 12-ton trucks on a 12 ft lane gave 27.9, 28.3 and 28.0 lb/yd² against a 28 lb/yd² design, an average of 28.1. It notes that aggregate moisture must be compensated for when calibrating spreaders. [2]

Maker literature shows how these functions are built. Etnyre's ChipSpreader brochure describes a computer that keeps aggregate application uniform through changes in road speed and stores five application rates. It also lists individual one-foot power gates, a variable-width spread hopper in 18, 20, 22 and 24 ft systems that extends and retracts while spreading, 24 in. wide conveyor belts, a folding rear receiving hopper, a truck hitch and front-wheel or four-wheel hydrostatic drive. [3]

Give the supplier the lane widths and spreading width needed, aggregate size and target spread rate, truck types to be hitched, required rate control and gate adjustment, travel and working speeds, calibration method (for example the agency's specification or ASTM D5624), engine and emissions requirements, and transport dimensions.

02

Plan the chip-seal sequence around the spreader

FHWA's checklist says the aggregate spreader should follow closely, 100 ft or less, behind the distributor, travel slowly enough that the chips do not roll when they hit the surface, apply damp aggregate with emulsions and dry, pretreated aggregate with asphalt rubber or hot-applied binders, and stop if the binder covers the chips or if streaks or plug-ups appear. [1]

Rollers should follow closely behind the spreader: the first pass within two minutes of applying the aggregate, pneumatic-tyred rollers recommended, rollers in echelon to cover the lane, rolling at 3 mph or less, with at least three passes for emulsion and five for asphalt rubber or hot-applied binder, and rolling completed within 15 minutes. Haul trucks should stagger their wheel paths when backing into and leaving the spreader. It advises avoiding work when wind speeds exceed 20 mph. [1]

Editorial calculation: 100 ft is about 30.5 m and 3 mph about 4.8 km/h. [1]

03

Calibrate the spreader on site with rock lands and a test strip

The Texas Department of Transportation (TxDOT) Seal Coat and Surface Treatment Manual says its specifications require aggregate spreaders to be self-propelled with a continuous feed, and names six major components: truck hitch, receiving hopper, belt conveyors, spreading hopper, discharge gates and discharge roller. The haul truck is towed in neutral by the spreader, because the application rate depends partly on travel speed and the spreader must regulate its own speed. [4]

TxDOT checks the rate with rock lands: the area one predetermined truckload covers at the design rate, marked on the pavement or with flags before work starts. The manual specifies the rate as cubic yards per square yard and gives the example of a 12 ft lane, a 14 cubic yard truckload and a rate of 1 CY/125 SY. If the aggregate runs out before the marker, the rate is too heavy; aggregate left at the marker means the rate is too light. During spreading, if the second truck is hooked up more than 25 feet before the first rock land marker, the aggregate is being applied too heavily and the gate openings are adjusted. [4]

Editorial calculation from the TxDOT example: 14 CY × 125 SY/CY = 1,750 SY = 15,750 sq ft; divided by the 12 ft lane width, one rock land is 1,312.5 ft (about 400 m) long. [4]

Before asphalt is applied, the manual allows a test strip on bare pavement with enough aggregate in the spreader for about a 50-foot run, so faulty gates can be corrected. In the visual check, the curtain of aggregate dropping through the gates should be one particle thick with light visible through it; dark streams mean a gate is open too wide, and alternating thick and thin transverse streaks (a ripple effect) mean the spreader speed is too high. The discharge roller is checked for warping with a string line stretched from end to end. [4]

04

Metong's chip spreader listing

Zhejiang Metong Road Construction Machinery Co.,ltd. lists chip spreaders on its website.

Ask any supplier for spreading width and gate layout, rate control system and how it compensates for speed, hopper capacity and conveyor design, spread roll and screen, truck hitching, drive and travel speeds, engine and emissions, calibration support, and transport dimensions and weight.

05

Hopper, conveyor, gates, spread roll and rate control

06

Chip spreader data the specification should pin down

Width, gate layout, rate uniformity, rate control and drive can be filled from Etnyre's brochure, AASHTO's Section 406 and the FHWA and TxDOT guidance; hopper capacity and spread-rate range are not given in the sources and must come from the supplier.

07

Send the road, aggregate and crew data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Roads: [lane widths; lengths]
Aggregate: [size; target rate]
Binder: [emulsion; asphalt rubber; hot applied]
Spreader: [width; gates; rate control]
Trucks: [type; hitching]
Calibration: [method; ASTM D5624]
Speeds: [working; travel]
Transport: [dimensions; weight]

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