Constant Pull vs Lockout Stringing Machine: What Changes

Last checked: August 2026 · Written by the player-led team at Badminton House

Two shops can both hand you a racket at "24 lb" and still give you back different rackets. Most of that gap comes from what the tensioning head was doing during the pull, not from the number on the ticket: a constant-pull head keeps correcting the string as it stretches, while a lockout or crank head pulls once, stops, and locks.

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What the Tensioning Head Does During a Pull

A tensioning head does one job on every pull: apply force to the string until a target number is reached, then either keep correcting for movement after that point or stop and lock. That single design choice, more than the number on the ticket, decides how close the finished bed lands to what you asked for.

An electronic constant-pull head "pulls the string and continues pulling until the desired tension is reached," and keeps the string "correct at all times" as it stretches during the pull, according to the reference explanation of stringing machine mechanisms. In practice that means the motor doesn't just hit the number and stop. It watches the string relax slightly under load and tops the pull back up, right up until the operator clamps it off.

A GAMMA 7500 Els, a widely used electronic constant-pull machine, shows this directly in its own manual: the tensioner "start[s] pulling at full speed and then slow[s] down as the tension in the string approaches the tension setting," and once it hits that number the string is simply "ready to clamp off," per GAMMA's owner's manual. It holds and doesn't release until you act. That same machine's tensioner spans roughly 10 to 90 lbs (about 4.5 to 40.8 kg), which gives a sense of how wide a service range one constant-pull head can cover.

A lockout head works differently once it reaches target. It "will pull to the desired tension but no longer monitor any reduction in tension over time," and a manual crank machine is built the same way: the crank brings the string to the number, "at which point the tension mechanism locks into place." Crank machines also show more measured tension loss than drop-weight machines built to hold a constant pull, per the same source. Crank and lockout heads belong to one family here: both stop correcting once they hit the target.

A third family, drop-weight machines, actually sits closer to constant-pull than to lockout. They use only a hanging weight and the mechanics of torque to hold tension, which the same reference describes as "very precise and also constant pull type of machines," since gravity keeps applying the same force to the string for as long as it hangs there, the same job an electronic constant-pull head does electronically. You'll mostly find drop-weight machines in older club setups and self-service stringing rooms. The physics is different from an electronic head, but the practical effect during the hold puts it in the constant-pull camp, not the lockout one.

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Constant Pull vs Lockout and Crank: What Changes

Constant-pull heads keep working after they touch the target number; lockout and crank heads (the same family under different controls) stop the moment they touch it. That gap in behavior during the hold, not any inherent superiority of one machine type, is what separates the two families in practice.

Picture two shops stringing the same racket to the same 24 lb request. Shop A runs an electronic constant-pull head and clamps within a couple of seconds of the target beep. Shop B runs a manual crank machine, and the stringer pauses to reposition the racket before clamping off. Both technically hit 24 lb at some point during the job. What comes back can still feel different, because Shop A's head kept correcting for those extra seconds of relaxation while Shop B's did not, and the pause before clamping gave that string more time to creep uncorrected. Neither shop did anything wrong. The number on the ticket describes the target the head pulled toward, not a guarantee about everything that happened between reaching it and clamping it off.

None of this makes a lockout or crank machine a worse choice by default. A careful operator on a crank machine, clamping quickly and consistently, closes most of the gap a constant-pull head would have corrected automatically. What the machine type actually buys you is fewer variables to manage, not a guaranteed better bed.

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What Pre-Stretch Actually Changes

Pre-stretch is a separate function from the head type: it pulls the string above your requested tension by a set percentage, releases it, then re-pulls it back down to the actual target. It is available on constant-pull and lockout machines alike, and it changes the initial bed more than it changes what happens weeks later.

On a GAMMA 7500 Els, pre-stretch "pulls string 10% or 20% over the tension setting... releases the string, and repulls to the tension setting," toggled per pull, per the same GAMMA manual. The idea is that overstretching first burns off some of the string's own elongation, so the string that ends up in the frame has already given up some of the stretch it would otherwise lose during your first few rallies.

One instrumented comparison backs that up only partway. In a test of a matched pair of rackets, one strung with a 10% pre-stretch and one without, the pre-stretched racket measured noticeably higher right after stringing, "almost 30 Hz higher" on the frequency-based tension reading used in the test, according to an independent stringer's tension-retention test. That gap is closer to roughly a pound of extra pull than the two-pound bump some players expect. More importantly, over the following days the two rackets "behave[d] identically... in terms of tension loss over time." The same test found no measurable difference in how well tension held, and no basis for a claim that pre-stretch shortens string life; it simply was not what the data showed.

The practical read: pre-stretch buys a slightly tighter bed on day one, not a bed that holds tension longer. If your machine doesn't offer it, asking for roughly a pound more on the reference number gets you close to the same starting point, per that same test's own conclusion.

Because the effect shows up immediately rather than building over time, pre-stretch matters most if you play your first session within a day of stringing and want the bed closer to target from the first rally. It matters less if you always let a racket settle for several days before using it: the same test found no lasting gap between the pre-stretched and non-pre-stretched strings once tension had a few days to settle.

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Does It Actually Matter to You, and What to Ask

For most club players, the machine type matters less than consistency: the same stringer, on the same machine, stringing you the same way each visit, beats chasing a specific head type. It matters more if you restring often, compare setups across shops, or notice your racket feeling looser than expected within the first week.

• If you restring at one shop and rarely compare, the head type is close to irrelevant. Judge the finished bed, not the machine label; the post-restring quality check covers what to look for.
• If you switch shops or travel for tournaments, knowing the machine type explains why an identical number can feel different, and gives you something concrete to ask about instead of just requesting a different pound figure.
• If your racket consistently feels loose within days of a restring, ask whether the shop's machine is lockout or crank and how quickly they clamp after reaching target. That is a more useful question than asking for a higher number to compensate.

Two follow-up questions help you interpret whatever answer you get back. If a shop says lockout or crank, ask how quickly they clamp after the target is reached: a fast, practised clamp closes most of the gap a constant-pull head would have corrected automatically. If a shop says constant-pull, you can spend less time worrying about clamp timing and more on the string and pattern choices that also shape the finished bed.

Next time you hand over a racket, three questions cover it: State the tension you want on mains and crosses. Ask whether the machine is constant pull or lockout (crank counts as lockout). Ask whether they use pre-stretch, and at what percentage, so you know if the number on the ticket already includes it.

None of these questions requires a shop to change machines. They just tell you which variables the equipment already handles and which ones depend on the person running it, so you know what to compare the next time the same tension feels different.

Banner photo: Clément Bucco-Lechat, Wikimedia Commons, CC BY-SA 3.0, cropped.