Development history: The development of HMT started in the 1950’s

The development history of Honda HMT (hydraulic mechanical transmission) goes back to the 1950’s. In those days, Honda was developing scooters to realize an easy-to-operate vehicle. As one of the many technical attempts, they researched infinitely variable transmissions, and recognized an infinitely variable, hydraulic mechanical transmission. After licensing the patent from the Italian machine tool manufacturer, Honda added unique ideas and applied the transmission to the Honda scooter in 1962. In the 1970’s, application of a belt-driven, infinitely variable transmission (belt converter) to scooters started. Honda continued to recognize the advantages of HMT, and kept on further challenges on enhancement of efficiency and reduction of size and cost along with automatic controlling of reduction ratio using the latest electronic control techniques. In an attempt to improve performance, the research and development started in 1985 for an application of HMT to motocross racing machines having high demands for transmission.

Basic patent licensed from Badalini, Italy in the 1950’s
In an attempt to realize an ideal transmission to apply to the scooter that was “easy” and “comfortable to ride”, and “helps to enjoy life” for everyone, Honda studied various types of infinitely variable transmissions. Among those, Honda recognized the performance of the hydraulic mechanical, infinitely variable transmission that Badalini of Italy had developed for machine tools (Cambi Idraulici Badalini). As the basic patent had already been granted when Honda started the development, Honda obtained a licensing agreement for the basic patent from Badalini, and started further research and development for an infinitely variable transmission for motor vehicles. The developed transmission was applied to the Honda scooter “Juno” marketed in 1962. In Honda, the transmission was casually called as “Badalini transmission”, and formally referred to as “HRD transmission” taking the initials of Honda R&D.
What is the hydraulic mechanical transmission (HMT)?
In the infinitely variable transmission using hydraulic pressure, there is the HST (hydraulic static transmission) that connects the hydraulic pump with the hydraulic motor only by hydraulic lines, and the HMT (hydraulic mechanical transmission) that takes out the driving force from the hydraulic pump via the output shaft. Generally, while the transmission efficiency of HST is not so high because the power is transmitted only by the hydraulic pressures, the system permits free layout of the output shaft and compact packaging, and is widely applied to construction machines, aircraft, power equipment, etc. Meanwhile in the HMT (hydraulic mechanical transmission), the hydraulic pump and the hydraulic motor are generally integrated into a unit, and the torque to drive the pump is added to the torque transmitted by the hydraulic motor. For that reason, higher transmission efficiency is attainable from HMT than from HST
HRD transmission applied to the scooter “Juno” in 1962
The “HRD transmission” was developed as a high efficiency, hydraulic mechanical, infinitely variable transmission. As an electronic control system was obviously unavailable in those days, the transmission was a manually-operated, infinitely variable transmission. The system was such that the left hand grip was used for shifting and the right hand grip for the throttle. The ‘HRD transmission” was a combination of a constant-volume hydraulic pump and a variable-volume hydraulic motor. With the hydraulic clutch that allowed bypassing of high and low pressures applied, the system permitted manual operation as well as automatic starting. The working pressure was considerably low at 6.68MPa.
Scooter "Juno M85"
Scooter “Juno M85”

Badalini type HRD transmission
Badalini type HRD transmission
Click the picture for an enlarged view

Motocross machine RC250MA won the all Japan championship series

As the automatic transmission HMT for motocross racing machines required high efficiency and reduced weight, while the principles were derived from the “HRD transmission”, the system configuration had evolved to a completely different, hydraulic mechanical transmission system with the pump and the motor laid on a common axis. The system was made more compact and light in weight by increasing the working pressure (normal working pressure: 44MPa, maximum working pressure 80MPa). Furthermore, the construction that prevents leakage of high pressure fluid was applied to various places in the unit, thus allowing the system to maintain the high transmission efficiency even under very high fluid pressures. One of the design features was the use of a spool-type distributor valve. Considering down-sizing of the control system and freedom of setting, the electronic control system was applied. The HMT for the motocross machine was aimed at an ideal, infinitely variable transmission that could respond discreetly to rider intentions, and accordingly it was named “HFT” (Human Fitting Transmission).

The motocross racing machine RC250MA equipped with the “HFT” won the all Japan motocross championship series in 1991, which was the second year since the debut. The “HFT” using very high pressures yet maintaining high efficiency was given intelligence through the electronic control system, and proved its potential in the real world of racing. Following the success in races, further research and development started for an application of the transmission based on “HFT” to mass-production products, and the development of HMT (hydraulic mechanical transmission) continued to the “Hondamatic” introduction in the year 2000.
Infinitely variable transmission HFT
Infinitely variable transmission HFT
Click the picture for an enlarged view


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