Tie bar nuts, end caps and lock rings

A tie bar nut is the large threaded nut on the end of a tie bar that fixes the bar to a platen and carries that bar's share of the clamping force. On a toggle machine, the nuts on the stationary platen hold the front ends of the four bars, and an end cap and lock ring over each nut stop it from turning. At the other end, behind the rear platen, a second set of nuts does a different job: they are the adjusting nuts of the mold height adjustment.

What the tie bar nut does

When the mold is clamped, each tie bar is pulled in tension. The nut stops the bar from being pulled through the stationary platen: the load passes from the bar into the nut through the engaged threads, and from the nut's face into the platen. Two things follow from this:

• The load is not shared evenly along the thread. The first few engaged threads carry most of it, which is why tie bar cracks so often start at the thread root next to the nut face.
• The nut's position on the bar decides how that bar is loaded. If one nut turns, its bar carries a different load from the other three, and the platens lose parallelism.

Types of tie bar nut

Designs vary between machine makers and machine sizes, so always compare a new nut with the old one:

• Solid round nuts with a fine thread, locked by screws, a lock ring or a cap.
• Split (clamping) nuts, which have a slot and clamping screws that pinch the nut onto the thread to lock it.
• Half nuts made in two pieces. On two-platen machines, half nuts close around grooves or threads on the tie bars to lock the moving platen to the bars before the clamping force is built.

The tie bar nut we supply is a split nut / half nut design in alloy steel, listed for Haitian MA and MA II series machines.

End caps and lock rings

The end cap and lock ring sit over the nut on the outside of the stationary platen. Their jobs are:

• to lock the nut so it cannot work loose under the load that rises and falls on every cycle;
• to keep the tie bar in its set position in the stationary platen;
• to cover the thread and keep out dirt, water and grit that would damage it.

See the tie bar end cap and lock ring we supply. If a cap or lock ring is missing, or its screws are loose, check whether the nut behind it has moved.

Signs of a loose or damaged tie bar nut

• A paint or witness mark across the nut and the bar that no longer lines up.
• A gap between the nut face and the platen, or fretting: fine reddish-brown powder or polished patches at the nut face.
• Flash on one corner of the mold, or a platen parallelism error that keeps coming back.
• Different load readings between the bars, on machines with tie bar strain monitoring.
• A knock or bang at the moment the clamp locks.
• Damaged, galled or flattened threads when the nut is removed.

Common causes are a nut that was not tightened or locked correctly after earlier work, a missing lock ring, overloading (for example a clamping force set above the machine's rating, or a mold height set too tight) and corrosion of the thread.

Checking and replacing a tie bar nut

Work on tie bar nuts changes how the clamping load is shared, so follow the machine manual or the manufacturer's procedure, which gives the tightening method and values. The general sequence is:

1. Open the mold, switch off and lock out the machine, and release the hydraulic pressure.
2. Before loosening anything, mark the position of the nut on the bar and on the platen, and measure how far the bar end stands out from the nut. This lets you put everything back in exactly the same place.
3. Remove the end cap and the lock ring or locking screws.
4. Unscrew the nut with a wrench that fits it properly. Large nuts may need the special tools the manufacturer specifies.
5. Clean the threads on the nut and the bar and inspect them for damage. This is also the best moment to check the thread root on the bar for cracks, for example by magnetic particle or dye penetrant testing.
6. Fit the new nut, lubricated as the manual specifies, and bring it back to the marked position.
7. Tighten and lock it, then refit the lock ring and end cap.
8. Measure platen parallelism, and the load on each bar if the machine can show it, before running production.

If the thread on the bar itself is damaged or cracked, the bar needs attention, not just the nut: our tie bar maintenance and replacement guide covers inspecting and replacing the bar.

What to send for a replacement quote

1. The machine brand, model and serial number.
2. Where the nut is fitted: stationary platen, rear platen (adjusting nut) or moving platen.
3. The thread size, pitch and hand, the nut's outside diameter and length, and its type (solid, split or half nut).
4. The number and size of the locking screw holes, and for end caps and lock rings the outside diameter, thickness and bolt hole pattern.
5. Photos of the old parts with a caliper or tape measure in view, and the OEM part number if you have it.

Request a quote with these details. For the rest of the assembly, see the tie bars guide.