Determining Lockup Speed

You've reading up on this eh? Lock-up speed about 53mph or equivalent why do you suppose that is?

If you are referring to the TCC, (Torque Converter Clutch), then the speed is determined by the weight of the vehicle, engine size, final drive ratio, transmission installed and the overall economy rating of the vehicle needed by the manufacturer. Some transmissions are designed to engage the TCC in 2nd gear, and others will not engage until 3rd gear. Most all will be required to be engaged when in over drive to prevent overheating the transmission fluid in the torque converter. In the early design transmissions they were designed to engage around 35 MPH. This created problems when driving at 35, (as this seems to be a common speed limit) because they would "shift too often" and cause the consumer to think something was wrong. They then increased the speed to around 43 for the transmissions that engage in 3rd gear and moved down to 27 for the transmission that engage in 2nd gear. The overall affect was that it reduced customer complaints and did not negatively affect the "Fleet Fuel Economy Rating" which is one of the major concerns for manufactures.

As for the second part of your question, "is the output not constant", the answer is determined by throttle position, road conditions, temperature, and a host of other inputs. The torque converter is simply a fluid coupler transferring rotational energy from an input shaft to an output shaft. The diameter of the converter as well as the internal parts of the converter, veins, stator, and spacing all have affect on the output. As the converter turns with the transmission fluid inside the fluid become "harder" or more dense from the centrifugal force. As the fluid begins to become more dense the fluid begins to turn the drive turbine. The fluid that is coming off the driven turbine is then redirected back toward the driven turbine by the stator thus giving it additional force. It never makes energy, it just make the available energy more useful. There are times under light to moderate throttle conditions and proper road conditions that the output shaft can be turning faster than the input shaft. I hope this help you understand what happens inside the converter.

Maybe this will allow you to answer your last question.

Correction: As the fluid begins to become more dense the fluid begins to turn the drive turbine. Should be DRIVEN Turbine

Sorry for the typo!!

I was with you until you started altering the density of the fluid. I don't believe that happens. You should look at the shear forces involved.

Yes Lynlynch, you are correct. Bad wording on my part. As the certerfugal preasure on the fluid changes the shear force increases. The drive turbine directs fluid toward the driven turbine. At a stop the converter has 100% slipage. At highway speeds the converter will have an average of 6% slipage. Of course this is without the TCC applied. When the TCC first came out, I learned that you could accellerate faster by applying a slight amount of presure to the brake. just enough to get the TCC to disengage. This allowed the tourque converter to do what it was originaly designed to do. Allow for more RPM without driving the vehicle at the same rate, affectivly reducing the gear ratio by 6-8%.

Thanks,

I was worried that I had slept through another Fluids class that was important.

You should look at the shear forces.

Every one knows you can't see the shear forces. They are all in camo, and blend into the surrounding areas. The shear forces are now deployed is the Arizona area for training.