Center Tie-Downs on Round Separators: What They Do, How to Measure the Stud, and the Tightening Order That Saves Screens

The center tie-down is the smallest assembly in a round vibratory separator. It stabilizes the middle of the screen so the mesh doesn't bounce, and on center-tensioned designs it also tensions the screen. Tighten it before the clamp ring or crank it down too hard, and the screen can tear around its center. Pick the wrong stud, and the assembly won't fit. This guide covers what the center tie-down does, how the two common designs go together, how to size a replacement stud, and the order Sweco says to tighten things in.

What to bring to the machine

• 7/8" and 13/16" wrenches (No-Tools assembly)
• Two wrenches for the jam nuts (LX18 to LX30)
• A soft-faced hammer and a torque wrench for the clamp ring
• A tape or rule, calipers and a thread pitch gauge, if you are measuring for a replacement
• Spare gaskets or cushion washers
• A spare tie-down assembly (Sweco's recommended spares list includes one)

What a center tie-down does

A round separator screen is held at its edge by the clamp ring that locks the frame stack together. On a large or fine screen, the middle has nothing holding it. The center tie-down is a threaded stud that comes up from the center of the frame below, passes through a hole in the middle of the screen, and clamps the screen's center between washers or a spacer and a cap. Its basic job is to stabilize the screen center. On center-tensioned designs it also tensions the screen.

Sweco's own explanation, from Jeff Dierig, writing as Sweco's global marketing manager in Powder & Bulk Solids, is that a center tie-down "is used to stabilize the center of the screen to limit any bouncing of the mesh when the separator is running." An earlier article by the same author describes it as reducing "the whipping action of the mesh." Kason says its reverse tension assembly "gives mechanical support at the center of the screen" and suggests it for high-capacity feeds. Describing a typical circular screener, a Kason engineer writes that "the washer and nut are tightened over the center post to tension the screen," and Midwestern says its "reversible center locking devices allow positive screen tensioning up or down."

Not every machine needs one. Sweco's LX manual marks the center tie-down as optional and notes that "the standard LX18 requires no center tie-down; must be specified if needed." Kason's clamshell lid, for example, is described as accepting screens "with or without center tensioning construction."

Names you'll hear for it

Each manufacturer has its own name for the same idea, which matters when you're reading a parts list or searching for a replacement:

• Sweco: "center tie-down assembly," often shortened to CTD. The threaded rod is the "center tie-down stud." In the shop it's usually called the all-thread.
• Kason: "center post" and "center plate," plus a "spring-type screen tension assembly" and a "reverse tension assembly" or "reverse tie-down."
• Midwestern: "reversible center locking devices," which its brochure says "allow positive screen tensioning up or down."

One name that does not belong on that list is "wagon wheel." In Midwestern's literature a wagon wheel is a screen support design for dense material on fine or large screens. It's a different part from the tie-down that clamps the center.

The two designs on a Sweco separator

Sweco's LX low-profile manual shows two tie-down designs, split by machine size. You need to know which one you have before you order parts or adjust anything.

How to tell which one you have:

• Hex jam nuts on top of a washer, and no cap: the stud-and-jam-nut type.
• A nylon cap on top, over a pedestal and spacer: Sweco's No-Tools type.
• On a Sweco LX, the nameplate model tells you: LX18 to LX30 use the jam-nut type, and LX40 to LX72 use the No-Tools type.

LX18, LX24 and LX30: stud and jam nuts

The smaller units use a stud with hex jam nuts, center tie-down washers and white neoprene cushion washers, one above and one below the screen. The parts list for the LX18 calls for two tie-down washers, five 3/8-NC stainless jam nuts and two cushion washers per assembly, with the stud ordered separately. You lock it with two wrenches. Hold the lower jam nut and tighten the upper one down against it.

LX40, LX48, LX60 and LX72: the "No-Tools" assembly

The larger units use Sweco's "No-Tools" center tie-down. It's a nylon pedestal and spacer on the stud, a jam nut, four white FDA neoprene gaskets and a nylon cap. The stud is locked to the table frame dome with a 5/8-NC stainless jam nut. Sweco's brochure describes this design as food-grade nylon suited to 220°F, and says it lets you "tension the screen from the top."

Sweco's No-Tools kit (part S48K00980) includes four gaskets. In a standard setup, one gasket goes above and one below the screen, and two go below the spacer. With a perforated plate, one goes above and one below the screen, and one above and one below the plate.

The cap has a left-hand thread. It tightens counterclockwise and loosens clockwise. The jam nut below it is a normal right-hand thread. If the cap won't come off, you're probably turning it the wrong way. Forcing it can break the threads on the cap and pedestal. Sweco's author notes that operators "will often attempt to force the cap onto the pedestal, breaking the threads on the components."

We could only reach Sweco's LX manual. Sweco's MX and HX machines may use different studs, so check your own machine's manual or parts list, or send us photos.

Measuring for a replacement: thread first, then length

Two measurements decide whether a replacement fits: the thread and the stud length.

1. The thread

The thread isn't the same across brands, or even across one maker's range. On a Sweco LX, the model does tell you: Sweco's LX parts lists call for 3/8-NC studs on the LX18, LX24 and LX30 and 5/8-NC studs on the LX40 through LX72. For anything else, measure the old stud's diameter with calipers, then confirm the threads per inch with a thread pitch gauge. 3/8", 1/2" and 5/8" are easy to tell apart.

Our tie-down assemblies are designed for machines with 5/8"-11 threaded nuts, which is the 5/8-NC size. Some machines have a 1/2" or 3/8" nut instead. If you have one of those, call us for availability before you order.

2. The stud length

If the old stud is intact, the simplest route is to measure its thread and overall length, or order by the part number in your machine's parts list. If you don't have the old stud, use our fit rule for our assemblies. This is ScreenerKing's rule for sizing our own studs, not an OEM rule:

1. Put the screen in place on the frame it ties down to.
2. Measure from the welded center nut up to the top of the screen surface.
3. Add 2". That's the maximum stud length. Choose a stud at or below that number. It still has to be long enough to pass through all the hardware and fully engage the cap or the top jam nut.

For example, 3" from the nut to the screen surface means a stud no longer than 5".

Not every machine has a welded nut at the center. On Sweco's LX40 through LX72, the stud is locked to the table frame dome with a jam nut. A Kason engineer describes the center post fitting a welded bushing. Measure from the top of whatever the stud threads into.

Don't assume one length fits every deck on the same machine. Sweco's LX parts lists show one to three standard stud lengths per model, plus, from the LX24 up, a longer ball-tray stud. That ball-tray stud is 11.5" on the LX40 through LX72, against standard lengths from about 4.75" to 9". Our 10" and 12" studs are listed for tall frames such as scalping frames. Measure each deck, and write it on your deck-by-deck screen spec.

The order that saves screens

This is the part that gets screens torn. Sweco's LX manual prints it as a caution:

"Always tighten clamp ring before tightening center tie-down assembly. This will help assure proper vertical location of screen center and help prevent premature screen failure."

The same Sweco author explains the reason in Powder & Bulk Solids: "If the CTD is tightened prior to the clamp ring, the mesh around the CTD will weaken as the clamp ring is tightened." Our reading of why: the clamp ring pulls the frames together and seats the screen's outer ring, so a center that is already locked down gets pulled on as the ring tightens.

Taking it apart

1. Shut off product flow, and lock out power.
2. Remove the tie-down parts above the screen.
3. Remove the clamp ring.
4. Lift off the upper frame.

Putting it back together

1. Set the new screen in place. Sweco warns not to let the screen "come in contact with the center tie-down stud" and get damaged on the way down.
2. Refit the upper frame.
3. Seat the clamp ring, tapping it with a soft-faced hammer, then torque it to the manufacturer's value. Sweco's LX manual gives 5 ft-lb (7 Nm) for the 18" ring (1/4" hardware) and 32 ft-lb (43 Nm) for the 24" through 72" rings (3/8" hardware). See our clamp ring guide for more.
4. Only then refit and set the center tie-down.
5. With everything back together, run the machine for no more than 10 minutes. Stop it, lock it out, re-check the clamp ring torque and retighten if needed. Re-check it again after one hour of running. Sweco's manual doesn't say whether to loosen the tie-down for these checks. If you had to retighten the clamp ring, check that the screen center is still level with the ring, which is also on Sweco's daily list.

These are Sweco's steps for one screen. We couldn't find a published Sweco procedure for stacking tie-downs through a multi-deck machine. A Kason engineer's description of a typical screener has a post through each screen's center. Check your machine's manual, or send us photos.

Kason's procedure for its reverse tie-down follows the same order: refit the clamp ring, then raise the reverse tie-down until the screen is evenly tensioned, then lock the jam nut.

How tight is tight enough

We couldn't find a published torque value for center tie-down hardware from any manufacturer. What the manuals give you is a target height and a feel:

• Sweco No-Tools (LX40 to LX72): Adjust the pedestal with a 7/8" wrench until the spacer touches the screen center disc. "The screen center should be level with the outer tension ring." Then "lightly tighten" the jam nut with a 13/16" wrench to lock the pedestal, and hand-tighten the left-hand-thread cap.
• Sweco stud and jam nuts (LX18 to LX30): Fit the cushion washer, tie-down washer and two jam nuts over the screen. Hold the lower nut with one wrench and tighten the upper nut down onto it with a second.
• Kason center post, at room temperature: Kason's own procedure at room temperature is to press the center plate down with two fingers, run the nuts down until they stop, then turn the tension nut another 180° with a wrench and tighten the lock nut. That's Kason's procedure for Kason's center post. Don't apply it to other machines. For high or changing temperatures, Kason recommends a spring-type tension assembly instead.

On Sweco's tie-down, level matters more than force. Sweco's brochure says the No-Tools design lets you tension the screen from the top, but the adjustment target in its LX manual is a level screen center, not a tension setting. Sweco's author warns that "if the screen is too convex or concave, it could add additional tension on the screen mesh and cause failure." On a Sweco machine, if you pull the center down until the screen dishes, you've added stress the screen wasn't built for.

Center-tensioned designs are different. Describing a typical circular screener, Kason engineer Ryan Long writes that "the washer and nut are tightened over the center post to tension the screen," and adds: "Be sure to draw tension to the center post. This is especially important for fine mesh screens." Midwestern's center locking devices also tension the screen. On these machines the center hardware sets screen tension, so follow that maker's procedure. The risks run both ways: Kason warns that over-tightening "can cause the screen wires to stretch - sometimes to the breaking point," and that too little tension lets the wires flex until they fatigue. For how screen tension affects separation, see screen tension and separation efficiency.

Sweco's daily checklist covers the tie-down too: check it for wear or damage, check that it's level with the screen, and check that it's tight.

Ball trays, sliders and perforated plate

Self-cleaning setups change the tie-down. With a ball tray or slider kit, the stud passes through the support below the screen as well as the screen itself:

• On the LX18 through LX30, Sweco's slider-kit drawing shows the stud going through the perforated plate, with stacked tie-down washers, cushion washers, jam nuts and a spacer clamping both the plate and the screen. The stud isn't part of the self-cleaning kit (the LX30 drawing labels it "not part of self-cleaning kit"), so order it separately.
• On the LX40 through LX72, the No-Tools assembly goes through the plate, with a gasket above and below the screen and above and below the plate. The LX72 uses a slider support screen instead of a perforated plate.
• On the LX40 through LX72, Sweco lists a separate center tie-down assembly for ball trays, with its own nylon spacer and pedestal. It uses the longer 11.5" stud, which isn't part of that assembly. Sweco's ball tray assembly includes it, or you can order it on its own.
• On the LX18, the self-cleaning kit's center stack calls for three tie-down washers, four jam nuts and three cushion washers.

Sweco's recommended spare-parts list includes this: "If unit includes a ball tray, we recommend stocking one center tie-down assembly (if applicable) for ball tray, one ball tray stud, and balls (specify material)." Our tie-down assemblies have an optional center spacer for perforated plate and slider ring setups, to keep the spacing even through the stack.

When the screen fails at the center

A tear or hole near the middle of the screen often points to the tie-down, but not always. Things that cause center failures:

• The tie-down was tightened before the clamp ring. This weakens the mesh around the center.
• The tie-down was installed wrong. Sweco's author: "If the CTD is improperly installed, this could lead to tears in the mesh around the area." Sweco's manual adds that if the jam nuts are "installed improperly or omitted," the tie-down won't hold the screen and the screen can fail.
• A missing gasket. On some machines, leaving out the gasket lets the screen's center hole hit the threaded rod. A Kason engineer writes that the repeated contact eventually knocks the center plate loose and damages the screen.
• The screen center isn't level. A domed or dished center adds tension, as above.
• Worn parts. Abrasive or hot service wears tie-down components. Sweco's author says that once the tie-down is compromised, "a full replacement is required," and suggests keeping a spare.

Two other causes have nothing to do with the tie-down. Sweco's author lists product mounding at the center, which is fixed by adjusting the lead angle, and feed slugging, which weakens the mesh where the feed lands. If you keep losing screens at the center with the tie-down set correctly, look at those next. For crack patterns at the edge and other failure signatures, see why round separator screens fail early.

Cleaning and food-contact service

Product can build up under the center hardware, so Sweco's cleaning instructions, for both dry and wet cleaning, say: "Always remove the screen gasket and center tie-down (if applicable) and check for damage or accumulations underneath the gaskets." Make the center part of every sanitation teardown, along with the gaskets and seals.

For material, Sweco builds its No-Tools tie-down from food-grade nylon with white FDA neoprene gaskets. Our tool-free tie-down is food-safe nylon with 304 stainless hardware. Our all-stainless version is 304 throughout, for abrasive service or where you'd rather not have polymer parts at all. For wet service, both can be fitted with neoprene washers for a water-tight seal, which also cut noise. Sweco says its own nylon tie-down is suited to 220°F. For hot service, call us before choosing a nylon assembly.

Material certs / Certificate of Conformance with alloy traceability available on request.

Ordering a replacement

Have these ready before you order a tie-down:

1. The thread size of the nut your stud threads into: 5/8"-11, 1/2" or 3/8".
2. The stud length for each deck: the old stud's length, or the part number from your parts list. For our assemblies, nut to screen surface plus 2" is the maximum. See measuring for a replacement.
3. Whether the deck has a ball tray, slider ring or perforated plate: if it does, measure with the tray or plate in place. Ball-tray decks usually need a longer stud (Sweco lists one from the LX24 up), and perforated plate or slider setups need the center spacer.
4. Wet or dry service: wet service usually calls for neoprene washers.

Not sure which design you have? Email a photo of the center of your screen with the hardware in place to support@screenerking.com and we'll identify it. If the photos don't settle it, we'll ask you to send the part in.

Prices are from our catalog as of October 14, 2026, and may change. The stainless assembly costs less than the tool-less one because it's wrench-installed without the no-tools nylon hardware.

Frequently asked questions

Should I tighten the clamp ring or the center tie-down first?

On Sweco machines, the clamp ring. Sweco's manual says to always tighten the clamp ring before the center tie-down, to put the screen center at the right height and to help prevent early screen failure. Sweco's author adds that if the tie-down is tightened first, the mesh around it weakens as the clamp ring is tightened. Kason's reverse tie-down procedure uses the same order.

How tight should a center tie-down be?

We couldn't find a torque value from any manufacturer. On Sweco's No-Tools design, set the screen center level with the outer tension ring, lightly tighten the jam nut and hand-tighten the cap. On a Sweco machine, a domed or dished center adds tension that can cause failure. On center-tensioned screens like Kason's, the center hardware is what tensions the screen, so follow that maker's procedure. Over-tightening can stretch the screen wire.

Why won't the center tie-down cap come off?

On Sweco's No-Tools design the cap has a left-hand thread, so it tightens counterclockwise and loosens clockwise. Forcing the cap, especially when threading it back on, can break the threads on the cap and pedestal.

How do I know what length stud to order?

If the old stud is intact, measure its thread and overall length, or order by the part number in your machine's parts list. For our assemblies, with the screen in place, measure from the welded center nut to the top of the screen surface and add 2 inches. That's the maximum stud length. Choose a stud at or below it that is still long enough to fully engage the cap or the top jam nut. Ball-tray and tall-frame decks need longer studs.

Does every round separator use a center tie-down?

No. On Sweco's LX line it's optional, and a standard LX18 has none. Kason's clamshell lid, for example, accepts screens with or without center tensioning.

Is a wagon wheel the same as a center tie-down?

No. A wagon wheel is a screen support design for dense material on fine or large screens. The center tie-down is the stud and hardware that clamps the screen's center.

Sources

• Sweco, Installation, Operation and Maintenance Manual for Low Profile Separator, Models LX18 through LX72, August 2012 (revised January 2015). This is the source for the clamp-ring-first caution, the assembly and adjustment steps, the left-hand-thread cap, the stud threads and lengths, the optional tie-down and LX18 note, the self-cleaning kit hardware, the recommended spares, the cleaning steps and the daily checks. It covers the LX line only. The copy we found is the Italvibras-vibrator edition, hosted by an equipment dealer rather than sweco.com. Check the manual that shipped with your machine, since part numbers and stud lengths may differ on later revisions.
• Sweco, Vibro-Energy Round Separators brochure (sweco.com). Source for the No-Tools center tie-down: food-grade nylon, suited to 220°F, tension from the top.
• Jeff Dierig (Sweco), "Troubleshooting Round Separator Screens," Powder & Bulk Solids, September 23, 2025. Source for tightening order, mesh weakening, level adjustment, tears from improper installation, mounding and feed slugging.
• Jeff Dierig (Sweco), "How to Best Care for Your Vibratory Round Separator," Powder & Bulk Solids, August 9, 2022. Source for whipping action, the left-hand thread, wear and keeping a spare.
• Kason, "How to Correctly Tension a Screen," Screen Tips, Vol. 6, No. 2, Summer 1991 (kason.com). Source for center post and plate, reverse tension assembly, Kason's tensioning procedure and over-tightening.
• Ryan Long (engineering, AMP, parent company of Kason), "Circular vibratory screeners: Five main causes of screen failure," Processing, February 25, 2022. Source for the center post, welded bushing and missing-gasket failure.
• Kason, Vibroscreen options page (kason.com). Source for the clamshell lid accepting screens with or without center tensioning.
• Midwestern Industries, MR Series Gyra-Vib Separator brochure. Source for reversible center locking devices and wagon wheel screens.
• ScreenerKing product pages, retrieved 2026-10-14. Source for the measuring method, thread compatibility, materials, options, prices and shipping.