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Why is the Milwaukee 1/2 inch Stubby Impact # 2555-22 struggling in this video?

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Why is the Milwaukee 1/2 inch Stubby Impact # 2555-22 struggling to break bolts free with the 4.0 battery in this video? Do you have to buy the 6.0 battery?

 

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2.0Ah Battery


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4.0Ah Battery

 

49JV08_AS01?$mdmain$

6.0Ah Battery

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Even with lithium ion batteries, if you're pushing a tool you'll typically notice some drop in performance as the battery drains.  In this video he has the lug torqued to 250ft/lbs which is the stubbys max nut busting torque, and "nut busting" is kind of marketing bs.  So to get 250ft/lbs you're going to need a fully charged XC battery and in the video his battery was down to 2 bars.

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A bolt torqued to 250 ft/lbs takes more than 250 ft/lbs to break it loose. Static friction vs kinetic friction.

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First, torque wrench accuracy on a calibrated torque wrench is +/-20%. Torque wrenches, particularly the click type, are notoriius for drifting. In an ISO 9000 shop you are supposed to recalibrate every 1000 turns which is about once a week fir the average mechanic.

Second, break loose vs. dynamic friction in a bolt is different. Try this. Tighten up a bolt. If you don't have a torque wrench go as far as you can. Wait 15 minutes. See if you can tighten it more. You will. The static friction argument says you can't but you will find out your bolt "loosened". About 80% of the torque used goes into overcoming thread friction and not tightening. So it's the same in both directions. But on top of that the bolt stretch relaxes among other things so you lose a lot of the initial holding even within minutes after tightening. But later on it can tend to seize up requiring more torque. So depending on a lot of not always predictable factors it can take more or less torque to loosen or tighten a bolt. The simplistic physics 101 class is totally wrong when it comes to bolts. There is a web site called boltscience that goes into a lot of this. That is why an inspector cannot simply take a torque wrench nd attempt to tighten already tightened bolts. This test doesn't work and that's exactly what the video is showing. In a word, it's bogus. A good test would be to use a NIST traceable torque sensor just like a calibration company uses. But since it pulses you need a computer to capture the peak to verify the results.

Third, these are brushless DC motors. It is not a brushed universal motir and more electronics is involved. The tool regulates voltage which is why as you get close to dead on the battery it still runs like full charge. At around 1-2 bars I've noticed some tools struggle a little but but above that it is the same torque. This is why it does not "bog down" and usually goes right up to the point that the battery is truly dead then just suddenly dies when the controller gives up. It's not like older brushed tools where it gets weaker and then grinds to a halt. Brushless DC is very different from a two wire universal DC motor and not just because it's stronger. Just Google BLDC motors. They are usually actually DC but no brushes or commutators and have to be fed with AC. Instead of a magnet (PMDC) the rotor is fed AC and it uses a rectifier on the rotor to convert to DC. Since fields consume very little power the main power path is to the armature (rotor) which in this case passes through power electronics to turn it into AC with current and voltge regulation and hence torque control and feedback. The tool cn probably destroy the motor if it didn't regulate the output. In the tool the controller has to convert DC into AC to feed the motor, and some are actually just electronically commutated AC synchronous motors which is truly AC altogether. So battery charge may affect it at the tail end but a couple bars of charge is easily enough for full power and that's one of the selling points.

Sent from my SM-T350 using Tapatalk

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