The payload maximizers: auxiliary axles may provide the ready-mix producer with the highest return at the lowest cost - Product Pulse - cement trucks

Concrete Producer, The,  August, 2003  by Don Talend

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In a low-margin, commodity-driven business such as concrete production, operations managers are trying to squeeze as much utilization out of their high-cost equipment as possible. The central-mix plant's daily capacity? Max it out (while developing a plan to handle returned concrete, of course). Those 10-yard mixer trucks? Pump 'em up to 12 and, by the way, get five trips out of 'em per day-not the four we're used to.

Often, when ready-mix fleet managers look at the truck components that will provide the biggest payback, sales managers effectively make the decision to purchase auxiliary axles for them. It's a lot more cost-effective to get more out of existing trucks than to buy more of them or build another plant in a strategic location to keep up with increased sales. So auxiliary axles represent potentially the greatest return at the lowest cost of any investment the fleet manager can make.

Rick Rickman, sales manager at McKinney, Texas-based Watson & Chalin Mfg. Inc., points out that auxiliary suspensions use one or, often, a combination of three types of axles: pushers, tags, and "stingers"-the kind that extends several feet behind the chassis.

Pusher axles are located ahead of a truck's drive axles. Tag axles, in contrast, are located behind the drive axles and offer an advantage over pushers, notes Rickman. For Federal Bridge Formula truck configurations, tag axles distribute more weight to the front axle or axles-which is usually a high priority with most of the truck's weight sitting behind the front axles-using the cantilever principle. Nearly all pushers and tags operate using an air-pressure valve and two sets of air springs and air bags. One set carries the weight and provides a cushioning force and another raises and lowers the axle.

Stinger axles are perhaps most responsible for making mixer trucks into the strangest-looking vehicles on the road. Despite this, they offer the advantage of providing greater axle spacing than tag axles while cantilevering weight forward to make it easier to meet Federal Bridge Formula requirements. According to the formula, axles located within a given distance of each other count as a single axle. In addition, drivers can deploy the hydraulically operated stinger axle's wheels only when the truck is loaded and on-road, reducing tire wear. Lifting the axle out of the way makes it easier to back up on the jobsite.

Steerable auxiliary axles typically are higher in initial cost than rigid (non-steerable) axles but offer lower maintenance costs because of relatively low tire wear and allow the truck to maneuver more easily. Their wheels' positive caster causes them to follow a truck's front wheels as the truck is turning. (Caster refers to the angle of the kingpin, which is positioned vertically into wheel-end pivots that steer the wheels. For forward truck travel, the top of the kingpin is positioned behind the centerline of the axle, creating positive caster. Negative caster occurs when the bottom of the kingpin is positioned behind the centerline of the axle to suit reverse travel.) A rigid axle drags the rear wheels across the pavement at a wider radius than the front wheels when the truck turns, causing tire scuffing. Steerables suit a longer truck that has a greater turning radius. Steerables normally have a slightly lower maximum rated weight capacity than rigid axles.

Without some kind of mechanical correction, steerable axle wheels' positive caster would cause the wheels to turn to the side as the truck backs up. Most lift axles raise the wheels out of the way using a signal sent from the backup lights to a solenoid that opens an air valve as soon as the truck goes into reverse. A less-common alternative for steerable axles is mechanical lockout, which essentially converts the steerable axle into a rigid axle using various means as the truck is backing up. Several manufacturers offer a third alternative: using the reverse signal to change the suspension's caster from positive to negative.

In recent years, several manufacturers have tried to help the concrete producer increase payload capacity further by reducing the weight of auxiliary suspensions. Composite plastics, for example, have a higher initial cost but can reduce weight by several hundred pounds compared with conventional materials, helping the producer to gain perhaps an additional cubic yard of concrete per truckload. Some manufacturers also have tried aluminum as an alternative material.

Below are several vocational suspensions and several truck chassis manufacturers' models that feature them. Configuring a mixer truck requires close interaction with your local dealer, so explore the available options in suspensions as you go about specifying a truck to suit your local market and your specific needs. For more information about these products, contact these manufacturers directly or circle their corresponding numbers on the reader service card.

COPYRIGHT 2003 Hanley-Wood, Inc.
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