Hammers and Axes

Hammers and axes swing an arm to strike the top of the opponent, which is often the weakest part of a robot. A hammer hits with a blunt head to dent and shock. An axe uses a pointed or sharpened head to pierce armor and, ideally, hit something important inside.[1] They're crowd favorites, and Team Run Amok notes they've become more popular in sportsman classes.[2] For a quick overview of every weapon type, see Weapons.

Safety: a swinging arm can break fingers even in a 150 g robot. Lock the weapon whenever the robot is out of the arena, and keep clear of its arc. See Safety and Failsafes.

Types

Overhead hammers and axes

The usual layout: an arm pivots at the back or middle of the robot and swings up and over to strike forward and down onto the opponent. Common ways to drive it:

Servo – the simplest choice in insect classes. It won't do much damage, but Ask Aaron points out that no beetleweight hammer is likely to beat spinners or strong wedges, so a quick servo hammer is a reasonable, easy option.[3]
Geared motor (brushed or brushless) – more speed and energy. Brushless motors without sensors can be hard to start smoothly under a heavy load like a hammer.[4]
Pneumatic – more powerful for the weight than electric, and used by many heavyweight axes. This wiki doesn't cover building pneumatic systems; see Safety.[5]

Many hammer and axe robots use a four-bar linkage or a gear or belt reduction between the motor and the arm (see four-bar linkages).

Spring- and flywheel-powered hammers

A small motor can slowly load a spring (or spin up a flywheel) and then release that stored energy in one fast swing. This gives a harder hit than the motor could manage directly. Ask Aaron's spring mechanism page notes that a "slip gear" release has enough range of motion for a hammer. These designs store energy that can be released by accident, so the SPARC rules on springs and flywheels apply (see Safety).

Hammer saws

A hammer saw puts a small spinning saw or disc on the end of the arm. It swings down onto the opponent and grinds into the top armor. They're seen in insect classes (Ask Aaron mentions beetleweight hammer saws such as Strikepoint).[6] NHRL's guide notes that saws on arms need precise alignment and time to cut through.[7] See also overhead spinning weapons on Weapons.

Horizontal hammers

A horizontal hammer swings sideways, like a door, across the front of the robot. These are rare. They avoid having to lift the arm against gravity, but they're harder to aim, and the hit pushes the opponent away instead of pinning it against the floor.

Thwack and reaction hammers

A thwackbot spins its whole body to swing an arm. An overhead reaction hammer flips the robot's own body forward with drive torque. These have no separate weapon motor, so many events don't count them as active weapons. See Thwackbot.

Strengths and weaknesses

Strengths

They hit the top of the opponent, where armor is usually thinnest.[8]
The arm can double as a self-righting mechanism by swinging all the way over and pushing off the floor.
Gyroscopic forces don't affect handling the way they do with spinners, and the robot can stay low and wide.
They look great to the crowd and to judges, because each hit is visible.

Weaknesses

Low energy. An electric hammer has only about half a turn to speed up, so it stores much less energy than a spinner of the same weight. Team Run Amok says a beetleweight electric hammer delivers much less destructive energy than other weapon types, and even a pneumatic one falls well short.[9]
Slow to hit again. After each swing the arm must reset, and a missed swing leaves you open.
Hard to aim. The opponent has to be in exactly the right spot when the head lands.
Recoil. Swinging a heavy arm quickly pushes the robot the other way, and can lift the nose or tip the robot over backwards.[10]
Many hammer robots can't fight upside down unless the hammer can right them.

Matchups

General guidelines only – see also Driving Practice.

Against wedges and lifters: a hammer makes a good partner for a wedge or forks. Get under the opponent, hold it, and hit it. Without that, a low wedge can get under the hammer robot.
Against spinners: hard. Ask Aaron rates beetleweight hammers as unlikely to win against spinners or strong wedges.[11] Timing a hit onto the top of a vertical spinner, or onto an undercutter that has been tipped up, is possible but takes practice.
Against robots with exposed tops (thin lids, plastic tops, exposed electronics): hammers and axes are at their best here.
Against TPU and other bouncy plastics: blunt hits get soaked up. One builder writing to Ask Aaron put it this way: TPU shrugs off blunt hits but cuts easily. That favors sharp axes and hammer saws over blunt hammers.[12]

Design considerations

Energy and gearing

Team Run Amok treats an electric overhead hammer as a spinner that only gets about half a turn to speed up.[13] The aim is to have the arm reach its top speed just as it hits:[14]

With too little reduction, the arm speeds up slowly and is still slow when it lands.
With too much reduction, it speeds up fast but tops out at a low speed.
Gearing changes the balance of torque and speed, but not the motor's power.

The free Team Run Amok Electric Hammer Spreadsheet (Excel) models a hammer swinging through 180 degrees (or more or less), and shows energy, time to strike and terminal speed. It can also model flippers.

Don't stall the motor

A geared hammer motor sits stalled against its stops at both ends of the swing. That draws heavy current and can burn it out. Ask Aaron describes two common fixes: drive the hammer from a spring-centered stick and let go at each end of the swing, or use a controller with limit-switch inputs that cut power at the end of travel.[15] Gear trains also break when the arm has nowhere to go. Builders have used toothed belt drives to take some of the shock out of the drive train.[16]

Servo hammers

At 150 g and 1 lb, a fast metal-gear servo is the common choice. Follow the same rules as for servo lifters: hard stops, metal gears, and enough current for the servo. If the hammer servo shakes ("jitter"), check its power first. Common causes are a servo powered above its rated voltage, a BEC that can't supply enough current, or a signal wire running too close to motors or the ESC (see Battery eliminator circuit (BEC)).[17]

Springs: be realistic

In one Ask Aaron answer about a spring hammer for a 150 g robot, Mark Joerger suggested aiming for a few joules, not the 100 joules first proposed. Releasing that much energy in half a turn would mostly flip the hammer robot over backwards.[18] Torsion spring energy is easy to estimate: energy = ½ × spring rate × angle² (rate in N·m per radian, angle in radians).

Structure

The hammer head is usually the heaviest, hardest part. Make the arm stiff, and support the pivot shaft on both sides with bearings.
Keep the center of gravity low and the wheelbase long so the robot doesn't hop or flip back when it fires.
Add a wedge, forks or a scoop at the front so the opponent ends up under the head and can't drive off. Some hammers clamp the opponent first.

How they win

Damage: a solid hit can break an opponent's top armor, electronics or battery. Judges score how much the damage affects the robot, not how dramatic it looks or how many sparks fly.[19]
Knockouts: damaging the receiver, battery or power switch stops the robot, which is then counted out after 10 seconds.[20] Under some event options, an exposed battery is a technical knockout (TKO).
Aggression and engagement: attacks count toward aggression and engagement even when they miss or don't cause damage, as long as you're driving at the opponent rather than waiting for it.[21] See SPARC Judging Guidelines.
Control: a hammer combined with a wedge or clamp can hold the opponent and push it into hazards (see Wedges and Control Robots).

Safety

Weapon locks: SPARC requires a clearly visible locking device on every moving weapon whenever the robot is outside the arena, and the lock must be in place when weapon power is switched on.[22] On a hammer this is usually a pin or bar that blocks the arm.
Springs and flywheels: large springs must be loaded and fired by the robot itself, remotely, and never loaded outside the arena or test area. Flywheels must not be spinning outside the arena. Both must fail to a safe position if radio or power is lost.[23] Plan how you'll discharge a cocked spring at the end of a match. The SPARC emergency procedures expect spring weapons to be released when there's a problem.[24]
Pneumatics: pneumatic hammers and axes fall under section 8 of the SPARC Robot Construction Specifications and any event rules. Ask Aaron describes them as much more complex and dangerous than electric systems.[25] Don't attempt one without an experienced mentor and the event's approval.
Test in a test box or arena, and check that the hammer stays still when the transmitter is switched off (see Failsafes).

Famous examples

Beta (Robo Challenge, Birmingham, UK) – a BattleBots heavyweight listed as a hammerbot. Ask Aaron explains how its electric hammer became competitive thanks to modern batteries, and notes the team's claim of about 3,000 joules per hit.[26]
Blacksmith (Al Kindle, HFA) – a BattleBots hammer robot.

Insect-class hammers are often servo-powered, as above, and sometimes geared or spring-driven.

Designs you can print

From the list on 3D Printing (links checked October 2026):

150g Hammer Battlebot (emancarrillo)
Absolutely Quackers – Team Panic's 150 g duck-shaped hammer robot
Servo Drive Hammerbot (150 g)
150g N20 Hammer/Flipper (JairEmia) – driven by an N20 gearmotor instead of a servo
150g Antweight Axe Mechanism – a geared arm for an axe or self-righter
ONI – a 150 g shuffler with a hammer saw (heavier than 150 g as built; shufflers get a weight bonus, see Multibots, Walkers and Shufflers)
AlexKorvin's budget training robot (3 lb) – a lifter that also comes in hammer and drum versions