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12-08-2013, 12:56 PM   #1
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Why can't screw drive be as fast or quiet as wave drive?

Just curious.

12-08-2013, 01:02 PM - 1 Like   #2
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Screw drive entails a mechanical drive train
from the in-camera motor out to the lens.
There are gears involved,
as well as the coupling between camera shaft and lens shaft,
not as direct as the wave drive.
Also, you're getting resonance
from both the camera body and lens barrel.
12-08-2013, 01:28 PM   #3
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I believe it can, but it probably isn't cheaper than to put a motor inside a lens.

Also, silent wave, Ultrasonic, Direct wave motor (except SDM ) etc. sounds much better than screw drive, which means all these fancy motors are good for marketing
12-08-2013, 01:58 PM   #4
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It can definitely be as fast, but all the unnecessary gearing is what causes the noise.


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12-08-2013, 02:31 PM   #5
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It also depends on the materials of the lens and gearing. The all-metal Limiteds are fairly quiet, while the mostly-plastic DA35 and DA50 are pretty shrill (the 50 more so).

There is also a AF/MF compromise in the design of the gearing: faster AF usually means shorter focus throw, which is a pain for manual focusing, while a longer throw for more accurate manual focusing means slower AF. Some lenses have a clutch and two different sets of gears, but that can add to size, weight, complexity, and cost.
12-08-2013, 03:39 PM   #6
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The engineers likely made a compromise between drive noise and space. They could easily make the gear set quieter by using helical-cut gears instead of straight-cut gears. Notice how a car with a manual transmission makes a whining noise in reverse gear compared to any forward gear: the reverse gear's teeth are parallel to the gear's axis while the rest of the gears are cut at an angle to the axis, usually 30. The downside to that is that the gears need to be thicker to provide sufficient engagement surface area to handle the torque.
12-08-2013, 06:18 PM   #7
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Great replies! Thanks for all the great information folks.
12-08-2013, 06:42 PM   #8
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Nothing wrong with screw drives. I have two FA lenses with screw drives and they work perfectly and fast. My SDM lenses either overshoot or seem to struggle to find focus like its under powered.

12-08-2013, 06:59 PM   #9
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Having taken my DA*16-50 apart several times, I can attest to the fact that there are gears there, too. I haven't taken a body apart, but I wouldn't mind betting that a piezo motor is used for the screw-drive. As aremmes says, the use of helical, rather than straight-cut gears would reduce noise generation, but the self-lubricating plastic gears are also a damn-sight quieter than metal gears, even brass.

Wave drive, like ultra-sonic drive etc, is just marketing terminology for a piezo-electric motor, by the way. The only exception seems to be the DC drive in the 18-135 and 560 lenses, but I can't find out what that is, precisely. "DC" implies "direct current" so it may be a DC stepper motor, which isn't piezo-electric, but which is electro-magnetic. Perhaps the screw drive motor is this type, too. Either way, it's the use of a pulsed voltage that allows the control system to determine how far the drive rotates each time, and thus know where the moving lens elements are.
12-08-2013, 08:05 PM   #10
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Ultrasonic motors use ultrasonic vibrations to generate torque, meaning it is beyond the limit of human hearing (~20000 Hz), i.e. it is literally impossible to hear their action. Any sound you do hear from them is associated with the (relatively) large scale movements of focusing elements.
12-08-2013, 08:25 PM   #11
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@aremmes

Not exactly related to the OP's question, but to your post.
  • Helical-cut gears are not necessarily quieter than straight ones, and helical gears don't need to be thicker than straight ones to handle the same torque - the contrary is true.
  • Noise from gear boxes is produced by gears whoose surfaces are not exactly envolute curves and/or have a slightly wrong distance to the next gear to which they are engaged. The required precision of form and distance is very high - engineers working on smaller gear boxes for tool machines told me they can hear it if the form error of the envolutes exceeds 10 microns (1/100 mm).
Background of all this is:
In a perfect gear box, at any given time gears are touching only along one line. This "touching line" is the only means to pass on torque. When the gears are turning, perfect gears would ROLL along each other, never scratch along (that's the reason for the unusual "envolute" curve of the gear surface)! As torque is passed only along the touching line (which ideally is of infinite small width), the only way to handle more torque is to make that line longer. Helical gears do that trick without making the gears thicker.

Making really quiet gears for the AF mechanism would cause a cost explosion for manufacturing AND quality control, also as the gears would have to be of hardened steel. Taking in account the small size of the construction, I guess the form error of the gears and the position of the gear bearings should not exceed 1/1000 of a millimeter - anything more you would probably hear.
Unfortunately, precision steel gears would produce less noise, but pass on existing noise much better than plastic material - the harder the better.

The reason why reverse gear of cars is louder than forward are cost-saving strategies. Gears are classified (and priced) by their precision, and no one will drive 300000 miles in reverse gear. Any of the high precision helical gears for forward may cost 100-1000 times more than the straight ones for reverse. The strongly reduced life (mileage) expectancy for reverse gear compared with forward is not important.

I remember that, roughly 25 years ago during my "productive life", I once wrote software for quality control of gears where the customers wanted to use standard metrology equipment (co-ordinate measurement machines) instead of dedicated and extremely expensive gear measurement devices. End of the eighties, starting price of good gear measurement machines was near to 1/2 million $ (including extended training for the operating engineers). Most of what I had to learn to successfully finish that job I have forgotten - just some basics are still there.
12-08-2013, 09:04 PM   #12
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QuoteOriginally posted by RKKS08 Quote
The reason why reverse gear of cars is louder than forward are cost-saving strategies
I agree with most of your post, but drivetrain NVH (noise, vibration and harshness) is a composite of many factors and the reason why the quality of gearing is related to the narrowness and length of the"touching line" is to prevent wear spots, excessive clearances and ultimately failure. The simplest way to reduce gear noise is to use soft materials (such as the composite or plastic used for engine timing gears), but that comes at the expense of durability. It seems to me that most of the noise from a screw-drive AF motor is from the paddle interface (the short bar that transfers motion from the camera body to the lens gears) which needs enough clearance to accommodate a bayonet mount lens being inserted and removed repeatedly. The noise comes from moving parts repeatedly contacting each other (slapping together); reduce the number of moving parts, and you also reduce noise.

Ultimately though, in lens AF motors are more expensive to manufacture than gearing to accept a camera's screw drive and require more space resulting in a larger lens body, which is also more expensive to manufacture, so screw-drive AF is very much a cost reduction strategy.

QuoteOriginally posted by Adam Quote
all the unnecessary gearing
I'm pretty sure if it was unnecessary, it wouldn't be part of the lens. Lenses aren't carved from a block of material, every piece that is added has a cost associated with it, and unnecessary gearing is an unnecessary cost that adversely affects the manufacturer's profit.
12-08-2013, 09:09 PM   #13
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QuoteOriginally posted by RGlasel Quote
I'm pretty sure if it was unnecessary, it wouldn't be part of the lens. Lenses aren't carved from a block of material, every piece that is added has a cost associated with it, and unnecessary gearing is an unnecessary cost that adversely affects the manufacturer's profit.
I meant that the gearing isn't necessary if AF can be accomplished using a ring motor (as Pentax does with DC lenses).

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12-08-2013, 10:36 PM   #14
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QuoteOriginally posted by Cannikin Quote
Ultrasonic motors use ultrasonic vibrations to generate torque, meaning it is beyond the limit of human hearing (~20000 Hz), i.e. it is literally impossible to hear their action. Any sound you do hear from them is associated with the (relatively) large scale movements of focusing elements.
Nonetheless, the physical elements of an ultrasonic motor use friction to clamp and drive the motor shaft, which will generate harmonics that can be in the range of hearing. The drive system incorporates shafts, bearings and gears, and these move at considerably lower rates than the drive impulses.
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