In some portion of industrial truck investigations, the cause of the crash can be linked to a failure in the braking system. Brake failure on vehicles in industrial or construction environments can lead to significant damages and injuries if a person is struck or run over, property is damaged, or the operator is injured.
In this article, Mechanical Engineer Stanley Jaworski, P.E. describes the types of braking systems used in industrial trucks, failure modes that can occur, and the requirements for maintenance, inspection, and safe operation.
Brake Failure in Powered Industrial Vehicles - Expert Article
The first step in examining a case of brake failure in a Powered Industrial Vehicle (PIV), also known as a Powered Industrial Truck (PIT), is determining what type of braking system is used in the incident vehicle. This not only helps identify the failure mode, but also the maintenance and inspection requirements for the vehicle in question.
Types of Braking Systems Used in PIVs
Band Brakes are brakes in which the braking material is attached to a metal band that encircles the outside of a drum or shaft. The braking action occurs when the band is tightened, usually by mechanical means, and the friction material makes contact with the shaft or drum. Band Brakes are found in some powered industrial vehicles, and parking brakes.
Drum Brakes are brakes in which the brake pads are located inside of the wheel hub. Operated by both mechanical and hydraulic means, the braking action occurs when the pads are forced outwards against the insides of the wheel hub by either mechanical linkages or by hydraulic wheel cylinders. Drum Brakes can be found on many forklifts.
Disc Brakes are brakes in which the brake pads are held in place by a hydraulic or mechanical caliper that when actuated squeeze a metal disc attached to the wheel hub, or in some cases a drive shaft thus creating the friction and braking power. Disc brakes are used in modern passenger vehicles, as well as certain forklifts and other small pieces of equipment.
Hydrostatic Braking is a byproduct of a hydrostatic drive system. A hydrostatic drive system uses adjustable swashplates within a hydraulic pump to drive the wheels of certain PIVs. When the swashplates are placed in a neutral position the fluid is unable to move, thus creating a braking action. While effective in slowing down and stopping equipment, most hydrostatic powered vehicles have manual emergency braking systems in place as well. Zero-turn mowers and most modern lawn tractors utilize a hydrostatic brake system.
Hydraulic Wet Brakes are, in theory, the same as a disc brake system. The difference is that there are several discs and several braking pads within the system. Hydraulic pressure is used to squeeze a friction disc pack. Every other disc is attached to either the housing in the axle or to the axle. When the pack is squeezed, the discs interact with each other to slow/stop the vehicle. These systems are normally encased within the axles of the vehicle and cooled by the lubricant in those axles. Forklifts and construction equipment typically utilize hydraulic wet brakes.
Causes of Failure
Wear and Adjustment
Aside from hydrostatic systems, all of the braking systems described in this article rely on wearable friction material. Wear on brakes is inevitable, because they function in a way that a pad (or band) presses up against a metal drum (or plate). However, these wear products typically degrade at a predictable rate, and in most cases can remain functioning at optimum levels with routine inspection and maintenance.
Foreign Object Damage / Petroleum Oils and Lubricants
The basic principle of braking systems is friction. The vehicle is stopped when pressure is applied to the wheel drum or disc by the brake pads or bands. The friction slows the rotation of the wheels and brings the vehicle to a stop. This friction can be lessened when oil from other mechanical parts of the vehicle contaminate the friction material. It is imperative that fluid leaks in any of the vehicle systems are identified and corrected as soon as possible, and that part of that repair process includes a thorough cleaning of any affected braking systems. Rocks, sand, cement and other items can become entrapped between the pads and drums or discs, which could lead to component damage or contamination that lowers the effective stopping power of the braking system. This is commonly referred to as FOD, or Foreign Object Damage.
Mechanical or Hydraulic Actuation
Pressure is applied to the braking components using both hydraulic and mechanical means. Wear causing excessive play, improper lubrication, and lack of repair to clevises, pushrods, and bell cranks can lead to ineffective braking or failure. Additionally, corrosion caused by water intrusion and dirt in hydraulic fluid can lead to brake line rot and failure, as well as pitting and leaks in master cylinders, boosters, wheel cylinders and calipers that can allow for leaks which may lead to poor braking and failure.
Cable Stretch and Adjustment
Many powered industrial vehicles have parking/emergency brake systems that are actuated by the means of a parking brake handle that engages the braking system via a cable. Over time, as brake material wears, users may need to pull the parking brake lever further in order to achieve the same level of braking forces. This behavior can stretch the brake cable to the point that the parking brake no longer receives the tension required to maintain pressure on the drum or disc, thus allowing for unwanted movement of the vehicle or lack of stopping power when the primary braking system fails.
Adjustments can be made in an effort to get the maximum life out of the brake pads or shoes. Braking becomes less effective as the pads wear, but adjustments to the brakes can tighten them back up. This is why inspections are so important: to identify when/if an adjustment should be made, or if the brakes have reached the end of their working life and need to be replaced.
Maintenance and Inspection
Powered Industrial Vehicles, like all equipment, should undergo a pre-operation inspection by a qualified operator. This inspection is the front line of equipment safety and is an opportunity for the operator to identify unsafe conditions and report them to appropriate maintenance professionals for repair. When a fault is found involving the braking systems of a PIV, the vehicle needs to be locked out to identify its fault and to prevent other operators from using it until the problem has been resolved.
In addition to pre-operations inspections, most manufacturers require maintenance based on calendar date, hours of operation, cycles of use, and based on condition and inspection. This maintenance is imperative to the safe performance of the machine, especially when it comes to the braking systems. The timing of routine inspection and maintenance will vary based upon the type of vehicle, the guidance provided by the manufacturer, and the type of braking system. Maintenance and inspection responsibilities fall to the owner, but the owner must be supplied those manuals by the manufacturer or seller of the equipment. Additionally, the manuals must be given to the end user in situations where the equipment has been rented or leased out.
PIV Brake Failure Investigations
When injuries or property damage has occurred as the result of a PIV brake failure, a forensic investigation would likely require a review of the maintenance and inspection procedures, and an inspection of the incident vehicle to determine the cause and timing of the failure. The mechanical experts at Robson Forensic have investigated incidents involving various types of industrial, agricultural, and construction vehicles and have likely done the work central to your case. The firm also boasts experts in complementary disciplines such as metallurgy, material science, and electrical engineering to address other related issues.
For more information, submit an inquiry or call us at 800.813.6736.
Stanley Jaworski is a mechanical engineer with a broad base of professional and vocational experience; his background includes more than ten years of engineering experience involving HVAC and industrial process systems, preceded by nearly ten additional years working as a welder and automotive technician. Prior to attending college Stanley worked in a series of auto shops where he attained ASE certifications in Brakes, Steering & Suspension, Engine Repair, and Engine Performance. He also became certified as a mobile air conditioning technician and became a proficient welder. His many years inspecting, maintaining, and repairing vehicles provides valuable insight when evaluating crash damage and reconstructing the complex series of events leading up to a crash.