free wheel tranny?

[Carolyn Phillip]

Anyone know if we will damage our Twin Disc transmission (on our GM 871), by allowing it to free-wheel without the motor running?
(While running a tug with a 1271 we always locked the shaft because we needed engine running to circulate oil on it).

[Pauls]

Contact Twin Disc. Why would you want to risk damaging a transmission based on advice from a forum? Also, Twin Disc has made a lot of different transmissions over the years. They will need a model number.

[DougR]

No one can answer that question without knowing the manufacturer and model of the transmission that you have. ie., Twin Disc, Allison, ZF, Capitol, BW, etc.

Typically transmissions that are free wheeled have restrictions on the length of time that free wheeling can be employed, largely because of heat buildup and lubrication issues. Frequently the requirement is to run the engine for xxx minutes per hour or per day to cool and lubricate the transmission. Check your owners manual for info that pertains to your specific transmission.

[PippaB]

Originally Posted by Pauls

Contact Twin Disc. Why would you want to risk damaging a transmission based on advice from a forum? Also, Twin Disc has made a lot of different transmissions over the years. They will need a model number.

What he said.

Twin Disc has a manual right on their website, up to you to determine if it covers your specific model. https://twindisc.com/wp-content/uplo...vL_0818_CD.pdf

Here is what it says:

3.7 Windmilling, Backdriving, and Towing
Backdriving occurs when an engine is shut down and the propeller shaft is being driven by the flow of water across the propeller. This is sometimes referred to as windmilling. The propeller shaft is rotating components in the marine transmission. Locking the propeller shaft in place will prevent backdriving.

Situations where backdriving may occur:
• Vessel being towed for any reason.
• Multiple-transmission vessel with one or more engines shut down while under way.
• Sailboat under way with auxiliary engine shut down.
• Vessel tied up or docked in heavy current.

Most current Twin Disc production marine transmissions (with the exceptions that follow) can be backdriven in the situations listed. Vessel speed under all towing or backdriving conditions must not exceed the normal propulsion speed of the vessel. The following backdriving (towing) options are applicable to all transmissions except MG-5170 series, MG-5300 series, MG-5600 serial, and all MG-6000 series (without the optional trailing pump). For these transmissions, only the instructions in b and c are applicable.

You need to employ only one of the following methods.
WARNING
Do not exceed normal vessel speed during backdriving. If oil sump temperature exceeds 100°C, damage will occur to internal components. If oil temperature increases to 100°C, reduce temperature either by reducing backdriving speed, or supplying additional cooling with water flow or by idling the engine.

a. Start the engine and operate the marine transmission in neutral at normal fluid pressures for five minutes, doing this once every eight hours. Maintain the backdriven marine transmission’s oil level at the FULL mark on the dipstick.
b. Lock the propeller shaft to prevent rotation.
c. Add a trailing pump to the lube circuit. See the hydraulic system prints for more details on auxiliary pump specifications for the applicable transmission or contact your Twin Disc distributor.
d. In the case of an inoperable engine, or pump failure, where pressure lubrication of the transmission is not possible; plug the dipstick tube and completely fill the transmission with oil. Prior to backdriving or towing, drain the oil down to FULL oil level. Repeat this process every eight hours.