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How small can the MD be made? What is the limiting factor?
The MD typically saves space in the axial dimension. The thinnest design to date is an 11mm wide, 600 ft-lb MD used in an automotive transmission. Saving space in the radial dimension depends on the relationship between the OD and ID. If the difference between the radius of the OD and ID is less than 16 mm then there isn't usually enough space for the strut.
The sample MD you sent me is noisy, will the MD be this noisy in the application?
Running dry, MDs will make a noticeable noise. When operating in the application, and lubricated as recommended, the MD is virtually noiseless. Noise tests were conductedon the MD by all of the Big 3 automakers. They concluded that there was no detectablenoise increase with the MD over conventional one-way clutches.
What is the maximum speed that MD's should be used at?
MD's are routinely tested and used in applications with operating speeds up to 15,000 RPM. Development programs are underway to develop high torque MDs for use at speeds up to 30,000 RPM.
What are typical stresses on the struts as compared to sprags?
High performance MDs are designed for 130,000 psi stress at a rated torque. This provides a safety factor of 2 relative to yield strength and a 1.3 safety factor for fatigue. Prototypes have been tested to well over 1 million load cycles at torques which produce 130,000 to 150,000 psi. Sprags are designed to operate at over 250,000 psi contact stress at rated torque. MDs convert 96% of stress to torque, 25% to axial force. Sprags convert 10% of stress to torque, 99% to radial force.
What is the best resolution that can be achieved with an MD?
The best resolution that we have achieved to date with an MD is 1.7 degrees. However, most applications do not require this fine of a resolution. Our experience with MDs in automatic transmissions has been that resolution, or rather maximum backlash, up to at least 6 degrees results in no detectable difference in shift feel or load carrying ability.
What determines the impact strength of the MD?
Differences in impact test results appear to be due to differences in strength and toughness of the various materials evaluated. Low density steel PM parts made from a mixture of elemental powders had the poorest impact results. As expected, parts made from high grade tool steel provided the best results.
How does the MD fail under excessive impact?
Failures in our testing have been outer case cracking. The cracks appear to start at a high tensile area near the strut point of impact on the notch plate.
Why is the snap ring doubly wound?
Since we often use the retaining ring for direct axial locating during overrunning as well as axial load bearing during lock-up, we wanted a retaining ring with a high axial load capacity as well as good planarity. So far, the two turn ring has been a satisfactory choice for these requirements.
What are the important design considerations for strut design?
The important considerations for strut designs are the load requirement and the overrunning and lock-up characteristics. Also, buckling strength is important because the size of the strut affects its behavior during overrunning and at the instant of lock-up. This aspect of strut design is discussed in SAE paper #960722. The theoretical considerations are reduced to a rule keeping the peak nominal compressive stress to less than 265,000 psi. This ensures that the strut can handle the peak stress of an application. Fatigue considerations may also influence strut size. For high speed applications, we confine our strut designs to similar lengths, widths and thicknesses so that our practical and testing experience will remain valid for the new design.
For proper functioning the MD needs to be filled with lubricant both for normal lubrication and to provide damping of strut motion during overrun. Lubricant flow through the MD is important for cooling during overrunning. For most applications flows as low as .15 gallons per minute are adequate. Proper lubrication needs to be determined for each specific application.
Can extreme pressure (EP) lubricants be used in the MD?
Yes. The MD is relatively insensitive to lubricant type. Presence of liquid around the strut during overrun is essential, however. We operate the MD's successfully using liquids as varied as gear oil and water.
How does oil flow through the MD?
In a typical torque converter application, during overrunning, the spinning notch plate functions like a centrifugal oil pump. The oil is drawn in at the bushing by this pumping action and is discharged past the retaining ring. During lock up, oil flow is not needed; however, some flow may be induced by pressure differences across the MD. In general, oil flow to the MD is a critical part of the system or subsystem design.
What is the relationship between oil viscosity and engagement time period?
Lock-up of the MD is a function of the strut end angles and the notch plate and pocket plate geometries. The strut is forced into engagement by the torque load at the instant of lock-up, so the lock-up function and the effective time of lock-up are independent of oil viscosity over a broad range. Low temperature testing has been carried out and shows that the MD fails to engage at temperatures where the oil viscosity is very high (nearly solid). The failure to engage temperature was -43F for ATF and -65F for helicopter transmission oil.
How much wear is expected due to free running? How will this affect the locking function?
Struts in the MD behave like the "shoes" in a hydrodynamic thrust bearing, so most of the time, they are held out of contact with the notch plate during overrunning by the oil. Long term overrun tests show wear on the notch plate and strut which ranges from slight polishing to non-visible or detectable. in properly lubricated MDs we have experienced no degradation of the lock-up function after extended overrunning.
How is the locking function affected due to wear of the notch plate?
We have not observed any change in the locking function as a result of the slight wear observed in some overrunning tests.
What about spring fatigue? How quickly will they wear out?
Spring life has been verified by several 1,000 hour overrun tests at moderate speeds and by a continuing full flexure test which is now past 30,000,000 flexures.
The MD operates based on strut engagement with geometric features on a notch plate. The exact instant of engagement for each strut depends on the exact dimensions of both strut and notch plate. There is no degree of precision sufficient to ensure that more than one strut will have exactly the same instant of engagement. Therefore, load sharing, by way of simultaneous strut engagement is unreliable and cannot be assumed for load capacity determination. However, load sharing can and does help in sizing components for fatigue life in those cases where fatigue strength is the limiting design factor. Design of an MD for nominal load sharing between two struts doubles the quantity of struts and springs or cuts the available resolution in half for a design with the same component count.
What are the side loads on the strut? How will they be supported?
As a practical matter, there are no side loads on the strut except for centrifugal forces. The MD was originally designed for use in high speed applications. For this reason, the axis of strut rotation is in line with centrifugal forces acting on the struts. In torque converter applications, the centrifugal load is not present because the strut carrier plate is chosen as the stationary member.
How much drag torque is experienced during free-running?
In overrun, the drag of the MD is comparable to a sprag running in an oil bath environment. We conducted tests and presented SAE paper #940730 on this subject.
How does the MD break down in an application of excessive torque?
The struts will begin to bend, leaving the plates intact but effectively "locked up" and not able to overrun.
Since only one strut engages will there be a radial load? Which part takes up the load?
Only one strut engages, so, during MD lock-up, there is a radial load on the bearing which is built in to the MD for overrunning. This is usually a hydrodynamic bearing and we select bearing size and material to manage the lock-up load as well as to manage incidental radial loads during overrun.
Each strut seems to experience a different life-cycle load. How will this be reflected in the durability of the struts?
Individual struts could experience different load histories; however, when used in applications where the lock up event is positionally indeterminate or random, like automotive torque converters or transmissions, the long term load histories for each strut are essentially identical. So, if we look at hundreds of thousands and millions of load events, the strut histories become the same.
What determines MD torque capacity?
MD capacity is simply the product of the tangential force vector sustainable by the strut and the radius at which this vector acts. Other considerations like the engagement angle and the effects of varying strut length, width and thickness have secondary effects on capacity.
How can torque capacity be increased?
Torque capacity can be increased by increasing the diameter of the MD and by using better materials. Axial length of the MD tends to be unimportant, or at least a weak contributor, to increasing capacity. When replacing existing sprags or roller ramps, we usually have sufficiently greater strength to match capacity using conventional sprag materials.
Powder metal parts have been evaluated and strength is better than that of equivalent sprag clutches. Several versions of the MD for automotive transmission applications have already been tooled successfully in PM.
What types of manufacturing do you recommend for different volumes?
For very low volume, performance critical applications, we recommend numerically controlled turning and milling. The cost is relatively high, but precision and part strength are very good. For low volumes (under 10,000 per year), we recommend investment casting. The unit costs are lower and precision and strength remain high. For medium volumes (tens of thousands per year), we recommend investment casting. Precision and strength are more than adequate for most applications. For high volumes (hundreds of thousands per year), we recommend the large MD parts be pressed, sintered, finished machined, and heat treated, powdered metal. The unit cost is low, precision is more than adequate, and fatigue strength equal to 75% to 80% of those of the very low volume process are achieved.
Is assembly of the MD more complicated/time-consuming than that of sprags or roller ramps?
The manual assembly of MDs is much easier than the assembly of a typical sprag cage, about the same difficulty as the assembly of a roller ramp clutch. This is based on the number of components as well as complexity. Automated assembly has been investigated with various assembly equipment suppliers and the difficulty relative to other types of clutches is similar to the manual assembly comparison.
While assembling a pocket plate and a notch plate, struts jump out. Is there a good method for this task?
Three things are critical for trouble free assembly. First, all of the struts must be centered in the pocket so that the edge of the strut does not get caught on the strut locating boss during assembly. Second, the spring must be in correct position so that it rotates down in to the spring cavity as the strut is forced down during assembly. Third, when the notch plate is assembled to the pocket plate, the journal bearing must be engaged straight and then the notch plate pushed down with no vibration or rotation. Following this procedure, and with a little practice, assembly becomes easy.
We estimate the total market at $1.5 billion U.S. dollars. Of that total market $1.2 billion is automotive; the remainder is in industrial, medical and aerospace applications.
What types of applications can the MD be used in?
The MD can be used in virtually every application in which a conventional one-way clutch us used and, since the MD is insensitive to vibration, some additional applications as well. Typical applications include automatic transmissions, torque converters, conveyors, industrial drives, auxiliary power units, and winches. The MD can also be used in several new applications which are not possible with conventional sprag and roller clutches including crankshaft pulleys, accessory drives, and rear axles. Additionally, we are working on developing an MD for a helicopter drivetrain that will operate at speeds up to 30,000 rpm.
What are the advantages of the MD over conventional clutches?
The MD is typically one-third the size and weight of currently available one-way clutches for a given torque capacity. It has a much longer life expectancy, can operate at extreme speeds, and assures positive mechanical engagement for critical "no slip" applications such as inclined conveyor backstops and helicopter rotors. the MD is generally insensitive to vibration and centrifugal force, lubricant type and operating temperature. For example, sprags and rollers can experience operating difficulties in turbine starter applications when the temperature is too cold or the turbine is running to fast whereas an MD will not experience these same difficulties because it is positive locking and insensitive to centrifugal force. Conventional one-way clutches can also have problems in applications where there are high levels of vibrations, which cause them to slip and wear prematurely; whereas the MD, being a positive locking device, will work and not experience wear problems.
Can we just buy one off-the-shelf to fit our application?
While several models are available, it is usually best to customize the MD for specific applications. In this way, application specific requirements can be achieved while taking advantage of the smaller size and weight of the MD.
Are you in production with any MD models yet?
Currently the MD is produced and sold for use in drag racing where its proven strength makes it a favorite with top racers. The MD is also used in Case Corporation torque amplifiers. The aftermarket is being served by Raybestos who is currently distributing MDs for GM4L60 transmissions. In addition, several high volume MD automotive applications are in production launch.