Why Powerful Drills MUST Have Auxiliary Handles

Heavier duty cordless and corded power drills and drivers often come packaged with auxiliary, or secondary, handles. Speaking from personal experience, auxiliary handles often make larger, heavier, and more powerful drills easier to balance, manage, and control.

But auxiliary handles aren’t just provided for user comfort, they’re there for increased user safety.

Until recently, I did not really think too much about this. It just made sense, that large/heavy/powerful drills should come with auxiliary handles.

A few months ago I learned about a new compact hammer drill that was nearing release. When talking with a product manager about the new drill, I found it curious that the mentioned the new compact drill was so powerful that it required an auxiliary handle.

When I asked a brand representative about why an auxiliary handle was required for a compact and relatively lightweight drill, they said that this was done to satisfy updated UL certification guidelines that will soon be going into effect.

Digging deeper, I was informed by a product manager that UL standards now require that drills exceeding a certain torque that are so below a certain physical size (height, diameter, etc) must come with an auxiliary handle. This didn’t really clear things up, so I reached out to UL directly and had a nice long chat with a very friendly engineer there.

Understanding UL and UL Standards

UL, Underwriters Laboratories, is a US-based global independent safety science company that certifies, validates, tests, inspects, audits, advises, and trains. One of their primary missions is to inspect manufactured products, including power tools, against a series of safety guidelines. Products that meet UL requirements receive certification.

Essentially, UL is an independent testing organization focused on consumer safety.

Generally, and this is my interpretation of all I have read thus far, if UL approves or certifies a product, it could be considered safe to use as intended.

UL approval means that a product was tested by UL experts and deemed to be safe to use.

That does not mean that products without UL approval are not safe to use. It just means that they are not certified by UL to be safe. For instance, I have a Micro-Mark drill press that is not UL certified, and understand that Grizzly Tools and other distributors of imported tools sell non-UL-certified tools as well. Certain imported products have CE markings, but that’s not the same thing.

With regard to the construction industry, OSHA 29 CFR 1926.302(a)(1) and 1926.403(a) requires that electrical tools and equipment be approved by a Nationally Recognized Testing Laboratory (NRTL), such as UL.

All electrical conductors and equipment shall be approved.

According to UL, there are no [United States] laws specifying that a UL Mark must be used. However, in the United States there are many municipalities that have laws, codes or regulations which require a product to be tested by a nationally recognized testing laboratory before it can be sold in their area.

Thus, while it is not compulsory for manufacturers to comply with UL safety standards and seek UL approval and certification, it is highly advisable.

Back to Drills

The UL modifies requirements as needs arise and in response to technological advancements. The latest modification to UL guidelines that govern cordless power drills was drafted in 2009 and put into effect Oct 7th, 2013. A UL safety engineer I spoke with has said that most manufacturers were already in compliance with the updated guidelines before it went into effect.

UL sometimes updates their guidelines to help harmonize US standards with international IEC standards and other export requirements. This way manufacturers don’t have to worry about designing their products to meet different sets of guidelines.

In terms of cordless drills and similar driving tools, the guidelines, including the requirement for auxiliary handles depending on power output and tool dimensions, is done to help prevent users from suffering unexpected injuries.

Holding onto a drill with one hand on the pistol grip and another hand on the auxiliary grip makes the drill much easier to control and stabilize in the case of recoil or reactive forces. For example, if a user is drilling into a wood stud and the bit binds or jams, the drill itself might twist around violently and potentially injure the user.

The UL guidelines are also there to help protect a variety of end users. Not everyone who uses power tools are tradesmen, industrial professionals, or DIYers. I have known artists, sculptors, scientists, and stage crews to use pro-grade power tools. UL-rated products are deemed safe to use by any user, regardless of body size or strength.

The UL Guideline for Common Power Drills

UL Guideline 60745-2-1, section 19.101 specifies that:

The force on the hand due to static stalling torque shall not be excessive.

Torque is a function of force and the distance at which force is applied. An auxiliary handle that is effectively longer than a drill’s pistol grip handle increases the reaction force that a user can oppose. It can help to think about lever arms and see-saws.

This information is provided for the purpose of editorial discussion. For the most accurate reference information, please consult official UL guidelines. The following discussion reflects my personal interpretation of the limited information supplied to me by UL.

Since I could not republish the images from UL’s guidelines, I picked out a suitable image of a Dewalt cordless drill to work with.

In the image above, I have drawn two lines, a and b, which denote two lengths – a: the length of the pistol grip handle, and b: the length of the auxiliary handle. Both lengths are to be supplied in meters, and are taken from the center of drill rotation to the bottom of the handles.

For the purpose of UL guidelines, 0.04 meters, or about 1.57 inches, is subtracted from a and b to yield L1 and L2.

To meet Ul guidelines, a cordless drill’s maximum reaction torque must not exceed 400 times the length of the handle, in newton-meters.

Consider a cordless drill with a 6-inch handle. This is 0.1524 meters. Subtracting 0.04 meters gives us an L1 length of 0.1124. Therefore, the maximum reaction torque must not exceed 44.96 NM, or about 400 inch-pounds in order to meet UL guidelines.

Let’s say the drill is designed such that a 7-inch auxiliary handle can be attached. This would be 0.1778 meters. Subtract the 0.04 meters to yield a value of 0.1378 for L2. As per the above equation, a maximum reaction torque of 55.12 NM, or 488 inch-pounds is allowable in order for the drill to meet UL guidelines.

The same guidelines apply to both corded and cordless pistol-grip power drills.

A Note About Maximum Reaction Torque

Cordless drills are tested by UL such the maximum reaction torque does not exceed a magnitude of 400 x the length of grippable handle area.

UL engineers measure a drill’s maximum reaction torque, which is the static stalling torque, by:

1. Connecting the drill to its rated voltage
2. Adjusting the mechanical gearing to the lowest speed setting
3. Setting any electronic control to its maximum speed setting
4. Powering the drill in its full “on” position

The mean measured value of stalling torque of a drill should not exceed the value calculated via the equations above and which are determined by the drill’s geometry.

Summary

I always thought that auxiliary handles were provided because two handles often make powerful drills more controllable. While this might still be true, the relationship between drill handle length, auxiliary handle length, and drill power is a lot more involved, at least for manufacturers that are seeking UL certification for their products.

UL-rated drills can still cause injuries, but if a cordless drill meets UL guidelines and passed certification, it generally means that the force on the hand due to static torque is not excessive, at least by their judgement.

I don’t mean to suggest that auxiliary handles are only provided to meet UL certification guidelines, as they do provide improved user comfort, control, and safety. Reaction torque is one of many things that UL looks at when evaluating power drills for certification.

I should point out that this all applies to cordless drills that have reactionary torque. Impact drivers and impact wrenches do not create reactionary torques, and thus this is why you don’t see compact impact tools with side hands.

While this article mainly focuses on auxiliary handles from a UL safety certification standpoint, I hope it prompts you to think about tool designs in general and how everything must be balanced.

This all also explains why power tool brands often engineer their new drills and drivers below a certain power/torque ceiling. Yes, they probably can design new drills capable of delivering over 750 in-lbs of torque, but not without taking main pistol grip and auxiliary handle lengths into consideration.