The Case of the Balky Brakes

DN Staff

July 15, 2009

2 Min Read
The Case of the Balky Brakes

A cost reduction leads to a faulty product

This case underscores the need to test when swapping in lower-cost components.

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The Scene of the Crime

A quality forklift built with a unitary frame, domestic hydraulics and an Asian-sourced engine and transaxle was priced below other domestic competitors, and sold well until rising parts costs killed any price advantage. To regain sales, the Asian transaxle was replaced with a lower-cost European unit and the truck was rushed into production with incomplete testing.

The Investigation

With two borrowed technicians, we ran brake tests following ANSI/ASME B56.1, Part III, Section 7.16 standards. A pedal force of 160 to 170 lb produced a brake drawbar equal to 25 percent of the loaded forklift’s weight, but the standard required a pedal force of only 150 lb, while our engineers typically used 125 lb as their design target. The killer was a 250 lb inching pedal force, evidently overlooked in tests.But why such high forces?

The Smoking Gun

For comparison, the techs pulled the wheels from the test truck and another truck with an Asian axle. I had an “Omigosh” moment as the brakes on the European axle measured three inches smaller in diameter than the Asian unit. I concluded that smaller brakes with the same master cylinder and brake pedal ratio, required higher pedal force.

Also, the designers replaced the original inching pedal with a 30-year-old design casting and heavy spring. The inching valve needed only modest pedal force, so I reasoned the spring took away force needed to apply the brakes.

I shanghaied a designer to make detailed layouts (pre-CAD days, you know) of the pedals, master cylinder and truck frame. Assured of pedal clearance, I revised brake pedal pivot points to restore proper pedal force.

I replaced the Victorian-era casting and its ingot-like spring with the original inching pedal linked to the brakes.

Shopping our production lines yielded an electric truck brake cable and a walkie forklift compression spring. I designed a bell crank to access the inching valve. As the designer documented the changes, the techs whomped up and installed the parts.

Tests showed the modified brake and inching pedal stopped the truck with authority. The service lads were wide-eyed with approval, and when the engineering manager and product chief engineer returned from vacation they pronounced the changes “a good engineering job.” Mandatory changes were made on all production vehicles and retrofit kits were installed on all trucks in the field. This model truck then regained its favorable sales position.

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Author Information:
Myron J. Boyajian, P.E., ([email protected]) is president of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are from his actual files.

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