NYC Transit solves frog problems with redesign: Frog problems not only caused accelerated wear, but also were responsible for noise levels loud enough to draw customer and neighbor complaints - New York City Transit

Railway Track and Structures, July, 2001 by Tom Judge

In New York City, where millions of people count on the subway 24 hours a day, seven days a week, dramatically-reduced service life for frogs accompanied by serious noise and vibration problems resulted in a Big Apple-sized headache for New York City Transit s track maintenance people.

Word began to come back that shortly after field installation of new frogs, severe "dish" formations began appearing on RBM frog points and wheel risers showed excessive wear. The resulting noise level was high enough so that customers started complaining.

NYCT has 1,600 mainline switches and 1,000 yard switches, most of them with #5, #6 and #8 frogs, so a potential system-wide maintenance and customer complaint problem loomed.

Antonio Cabrera, P.E., director track engineering, and Gustavo Gobbato, P.E., track design engineer, took on the problem for NYCT. They discussed the problem and the solution recently at Advanced Rail Management's Eighth Annual Rail/Wheel Interface Seminar in Chicago. They presented an earlier paper at the AREMA Technical Conference in Dallas.

To understand the nature of the problem, Cabrera and Gobbato conducted periodic field inspection and measurements of newly-installed RBM frogs. Specifically, they looked at three 115-pound RE frogs, two standard #6 and one standard #8. Of these, one standard #6 was installed on open-deck elevated track, while the other #6 and the #8 went into ballasted track. Four 100-pound ARA-B rail frogs, all of them standard #6 installed in Type TIM track (ties embedded in concrete), were also thoroughly inspected and measured.

"By observing the movement of the trains over the frogs, it could be seen that the wheels were actually airborne for a few milliseconds from the riser onto the point on facing moves, 'crash landing' and creating the 'dish' on the point," Cabrera and Gobbato said. "On trailing moves, the wheels were actually 'climbing' from the point onto the riser, crushing and wearing off the receiving surface of the riser."

To establish the path of the train wheels over each frog, the frogs were painted and a train rolled over them. The engineers took photos immediately thereafter. Also, Cabrera and Gobbato measured cross sections of the frog surface, including points and risers at the frog point and other adjacent locations to evaluate wear and geometry.

The two analyzed the path of wheels at their theoretical condemning limit while traversing a standard frog and analyzed the path of normal (average) hollow wheels in service while traversing a standard frog.

Besides reviewing the standard material specifications for manganese inserts, the two engineers researched the chronological evolution of the geometry of standard #6 NYCT frogs since 1915.

Cabrera and Gobbato reviewed the geometry of new manganese inserts of standard 10-pound ARA-B rail #6 frogs for compliance with NYCT's drawings and specifications and proposed design changes to the geometry of the point flangeways and wheel risers. They also modified prototype frogs using the new design criteria and installed them in the field for in-service tests and evaluation.

Geometry improvements

"NYCT standards for a #6 frog call for a 1-3/4-inch flangeway," Cabrera and Gobbato said. "We found that some frogs were manufactured with a 1-7/8-inch flangeway. A 1/8-inch difference in the flangeway may not seem excessive, but, in this case, is critical. NYCT frogs are mostly #5s, #6s and #8s, with a very-short length available for the wheel to transfer from the riser to the point and vice versa. A 1/8-inch increase in the flangeway width automatically creates a 3/4-inch reduction of the transfer length. With only a 1-1/4-inch of transition length available for a worn wheel at the condemning limit, it translates to a 60 percent reduction of the available transfer length."

They continued: "To further improve the transition of hollow wheel with a false flange up to 1/8-inch, the contact area between the wheel tread and the manganese insert should be increased. If the flangeway wall slope can be increased, the running surface for the wheel tread could be increased. This suggests modifying the top corner of the flangeway walls of the manganese insert to match the rail section. By doing so, a gain of 5/64 inches on the crown of the riser is achieved, representing a 15/32-inch increase of transfer length (37 percent of the available transfer length)."

Field observations showed that wheels were having side contact with the manganese insert in the throat area. The current NYCT Standard Drawing for #6 frogs did not specify the tolerance for the throat dimension, so some frogs were manufactured with throat widths of 2-1/8 inches instead of 2-1/4 inches. This side wear was not visible in frogs with 2-1/4 inch throats. To avoid this side wear, the throat dimension was changed to 2-1/4 inches, 1/8 inches, -0 inches.

Some frogs were manufactured with a 1/4-inch riser instead of the required 3/16-inch.

Eliminating riser

"This extra height of the riser has a significant contribution in having the wheels fall onto the point in the facing move, and to crash into the riser in the trailing move," Cabrera and Gobbato pointed out. "We, therefore, recommended eliminating the riser altogether. In addition, this elimination would facilitate its manufacturing and improve quality control."