A Systems Approach to Smooth Steering

The world's automakers have pretty much perfected the art of power steering for their vehicles. Smooth, precise and virtually effortless steering is the norm on everything from the smallest of the new breed of "city cars" to big SUVs.

It is a very different story in the world of articulated construction equipment such as wheel loaders, and yet another story in the world of material handling forklifts. Achieving anything remotely close to the automotive experience on these vehicles requires a lot of careful engineering and attention to detail because everything — from steering geometry to system components to vehicle dynamics — has to be accounted and compensated for.

Articulated "jerks"

In steering jargon, "jerk" is not meant as a derogatory word. It's an engineering term for what happens at the beginning and end of a steering maneuver on an articulated vehicle. These vehicles typically have a pivot point near the center, and often the operator is positioned forward of it. Steering is accomplished with a pair of hydraulic cylinders that literally "bend" the vehicle one way or the other to change its direction of travel.

Jerk occurs when the operator initiates or stops steering. It is measured as the derivative of acceleration in G/sec. The problem with jerk is that it is felt by the operator in the cab with 8 G/sec being a typical maximum value for mid-size and large equipment. Being subjected to an 8 G/sec jerk hundreds of times per shift can be a major factor in operator fatigue and creates obvious wear and tear on the equipment.

The traditional way to reduce jerk has been to install and include cushion valves or accumulators in the hydraulic circuit. This method is effective, but also adds cost and complexity to the steering system. These extra components simply divert some of the hydraulic fluid to the accumulator or take some fluid from the accumulator to smooth out the pressure spikes that are the actual cause of jerk. This approach treats the symptoms of the problem but does nothing to eliminate the root cause.

A better solution is to meter the rate at which fluid is added to, or removed from, the cylinders in real time to reduce the amplitude of the inevitable pressure spikes. This capability can be designed into the steering control valve by increasing the deflection angle between the spool and the sleeve to reduce the gain rate of the steering flow/pressure curve.

Gain rate is a measurement of the change in flow rate versus valve deflection. By increasing the deflection angle, by as much as a factor of four in the steering valve, it will take longer for the flow rate to reach its peak. Increasing the gain rate over a longer time period will now result in a lower absolute magnitude of the resulting jerk.

An example of this solution is the Wide Angle™ and cylinder damping "wide angle technology" incorporated in steering control valves manufactured by Eaton. By simply substituting a valve with Wide Angle technology for a conventional valve in a load sensing system, the jerk is cut by a factor of two or more with no other modification to the system. No additional accumulators or cushion valves are needed.

Eaton's Hydraulics Operations originally introduced Wide Angle technology on its Series 20 valves which are large, high-flow units intended for very large equipment such as articulated dump trucks and loaders used in construction and mining applications. The company recently has made this patented technology available on its Series 10 steering control units, which can be used on smaller vehicles.

This is a good example of the industry trend toward migrating proven technologies from large, high-end equipment to smaller, mass-market vehicles. Availability of Wide Angle technology on Series 10 steering control units makes it cost-effective to apply smooth steering to vehicles like log skidders and loaders. Since there are many more vehicles of this class in use, the total productivity impact of reduced operator fatigue will be much larger.