The CH-47 Chinook Forward Transmission

 

 

24 October 2012: Colorado Army National Guard members replace a defective forward transmission in a CH-47D Chinook helicopter in below-freezing temperatures during operation Maple Resolve at the Canadian Manoeuver Training Center in Wainwright, Alberta. Click-N-Go Here to view a larger version.

 

 

 

The Forward and Aft Transmissions of the CH-47D are similar in design and transmit torque from the Combining Transmission to their respective Rotary Wing Head. The torque is transmitted through a Spiral Bevel Ring Gear. At this point, the direction of torque is changed from horizontal to vertical, with the two transmissions driving the rotor system in opposite directions. When viewed from above, the forward rotor system turns counter-clockwise and the aft turns clockwise. The Spiral Bevel Ring Gear is bolted to the first stage Sun Gear. The Sun Gear, in turn, drives the first stage Planetary Gears which mesh with a Stationary Ring Gear.

 

Forward Transmission internal components. Click-N-Go Here to view a larger image.

 

The non-rotating (stationary) ring gear causes the Planetary Gears to revolve around the Sun Gear. The Planetary Gears are attached to the first stage carrier. The upper portion of the first stage carrier forms the second stage Sun Gear and it drives the second stage Planetary Gears. These Planetary Gears are attached to the second stage carrier and revolve in the same manner as the first stage Planetary Gears and carrier.

The second stage carrier is an integral part of the rotor shaft on the Forward Transmission. On the Aft Transmission, the carrier is splined to receive the Aft Vertical Shaft, as shown on the Aft Transmission page. The second stage carrier on the Forward Transmission and the top of the Aft Vertical Shaft are splined to receive their respective Rotary Wing Heads.

There are four first stage and six second stage Planetary Gears. The gears are manufactured from a chrome alloy steel. The gears are approximately six inches in diameter. Shown in the photograph to the right is a Second Stage Planetary Gear typical of either the Forward or Aft Transmission.

The overall speed reduction in the Forward or Aft Transmission is 30.72:1. Given the speed the Synchronizing Shafts are rotating at, 6,912 revolutions per minute (RPM), this drive reduction accounts for the Rotor RPM of 225.

The Forward Transmission changes the drive direction 81 degrees. The Aft Transmission changes the drive direction 94 degrees.

Aviation Unit Maintenance (AVUM) on either transmission is limited to performing removal and installation, minor seal replacement, cleaning, fluid changes, and other subordinate tasks.

Aviation Intermediate Maintenance (AVIM) on either transmission adds the tasks of Sump removal, repair, and installation of Sump related components. Note that observation of the first stage gears only, would be possible with these sumps removed.

All other maintenance on the transmissions must be accomplished at Depot, with very few exceptions on a case by case basis requiring specific instructions and approval.

 

A Boeing drawing that shows the tilt of the Forward Transmission set to 9 degrees forward.

 

Shown below are the Second Stage Planetary Gears in the Forward Transmission that are partly responsible for the August 1999 worldwide grounding of the Chinook fleet. During a transmission overhaul, a 145DS010-8 second stage planetary gear/bearing assembly in a British Royal Air Force HC-II (CH-47) was found to contain a crack in the spherical raceway of the gear. Subsequent metallurgical investigation determined the crack to be from a grinding process during the original manufacturing stages which had not been detected during final inspection of the product. Further inspection of other new first and second stage planet gear/bearing assemblies found additional second stage planetary gear/bearing assemblies with similar cracks. The Planetary Gears are the six smaller gears (first photograph) located around the larger Sun Gear (second photograph) in the center of the transmission.

Upper half of the CH-47D/F transmission. Click-N-Go Here to view a larger image.

 

 

Lower half of the CH-47D/F transmission. Click-N-Go Here to view a larger image.

 

 

To ease one's understanding of how these two pieces fit together, visually rotate the upper photograph to the right (clockwise) 180 degrees and place it on the lower photograph. The entire Forward Transmission then becomes apparent.

 

 

 

 

 

 

 

 

 

The CH-47D Chinook helicopter Forward Transmission Lubrication Diagram. Click-N-Go Here to view a larger version.

 

 

 

Above: A drawing showing the internal components of the Forward Transmission utilized on the D model and later versions of the Chinook helicopter. Numbers associated with a "T" indicate the number of teeth on a particular gear. Click-N-Go Here to view a larger image.

 

 

 

Above: A drawing showing the placement and operation of the drive shafts connecting the transmissions and the gears inside the various CH-47D transmissions. Upon close examination, one can see that there is not a rigid connection between the input and output of either the forward or aft transmission. The sun/planetary gear sections (upper portion of the transmissions) rotate around each other. From input to output in each component of the drive train there is large reduction in speed. This translates in to a tremendous increase in torque that is used to drive the rotor system. Numbers associated with a "T" indicate the number of teeth on a particular gear. Numbers associated with a rotational symbol is the speed that component rotates per second. The rotor system rotates at 225 revolutions per minute (RPM). 3.75 blades will pass the same point in space every second. Click-N-Go Here to view a larger image.

 

 

Preflight Items to Consider

 

During Preflight, often undetected are things that are difficult to observe. Take the time to bend over and stick your head into those hard to see areas and you might find a defect that, if corrected, could save your life.

 

For instance, an improperly routed line on the Forward Swiveling Actuator (from 89-00177 in 2002) can contact the transmission housing. After a few hours of operation, the line wears through and vents precious flight control hydraulic fluid overboard.

 

 

 

 

 

 

 

 

 

 

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