# What is the speed of hard disk actuator arm?

There is a lot of material discussing the rotational speed of the magnetic disk in HDD but not about the rotational speed of actuator arm.

What are the typical actuation speeds of the actuator in HDD?

This information will be used in the design of a fast shutter system made by salvaging the HDDD actuator arm.

• Look at the drive's track-to-track seek time, figure out the angle this represents and angular velocity can be calculated by angle/time. – Transistor Aug 30 '17 at 8:27
• Here is an article that shows the measurement of speed of the arms, as analyzed by slow-motion video. HOW FAST IS A HARD DRIVE ACTUATOR ARM? THE SLOW MO GUYS KNOW. Excerpt: >If you have ever wondered what goes on under the covers of a hard drive (and we know you have)…well wonder no more because the Slow Mo Guys were able to catch it on video…slow motion video. >The video is short and sweet, funny and shocking like what happens when you pour a gallon of water on a drive… Check it out. – SDsolar Aug 30 '17 at 8:36
• You can get the mean velocity from angle/time. Assuming constant actuator force -> constant acceleration, double it for the peak velocity. – Brian Drummond Aug 30 '17 at 11:32

You have two helpful measures for this:

track to track seek time and full stroke seek time.

track to track seek time will indicate how fast the head can move to an adjacent track on the disk. To get a velocity figure out of that, you need to know the track width.

full stroke seek time makes things a little easier, as it is the seek time from the innermost to the outermost track of the disk. With this value we can find an approximation without knowing all the details.

So this website claims a full stroke seek time of around 15 to 20 ms.

I approximated the length of the head assembly to be roughly 3 cm. And the length of the traveled arc approximated by a straight line is around 2.2 cm. So the traveled angle is 43°.

So the angular velocity should be in the order of 2150 to 2900°/s or 38 to 50 rad/s.

It could probably be faster if you don't need to position it so carefully like it is done in a HDD.

• Thanks. I got confused btw tracks and sectors. So couldn't relate. Anyways, I wish to position it with accuracy and hence trying hdd actuator. – karthikeyan Aug 30 '17 at 9:47
• @karthikeyan Keep in mind that the actuator is often assisted by servo patterns on the hard disk surface. It may not be able to achieve the same accuracy in the open air. – duskwuff Aug 31 '17 at 3:16