Vibrating Screen Support Structures: Designing and Building for Fatigue

A vibrating screen shakes on purpose. Everything underneath it shakes by accident. The support structure between the screen and the plant steel is where those two facts meet, and it is one of the most common fatigue failures in mineral processing plants.

This guide is for buyers specifying or sourcing screen support frames. It covers the load, the resonance problem, the weld details that decide fatigue life and the items that belong on the fabrication drawing. The mining equipment frames page covers the wider family of structures this belongs to.

The load is not the weight

A screen weighs what it weighs. The structure under it, though, sees the dynamic force from the exciter, applied at the operating frequency, typically 12 to 16 Hz for a large screen. Screen manufacturers publish the dynamic load at the support points as a force amplitude in each direction. That amplitude, multiplied by the number of cycles over the plant’s life, is the design case.

Two numbers make the point. A screen running at 15 Hz completes about 1.3 million cycles a day. A fatigue category that survives 2 million cycles at the design stress lasts less than two days. The structure has to be designed so the stress at every weld toe is well below the endurance limit for its detail category, not just below yield.

Resonance and isolation

Screens sit on springs or rubber buffers. The isolation is designed so that the screen’s operating frequency is well above the natural frequency of the screen on its springs, which means most of the dynamic force stays in the screen body. What passes through the springs is still large, and it still arrives at the support frame at the operating frequency.

The support frame then has its own natural frequencies. If one of them is near the operating frequency, the frame amplifies the load instead of carrying it. The usual rule in published design guidance is to keep the support structure’s natural frequencies at least 20 to 30 percent away from the operating frequency and its first harmonic. That is a design task for the plant engineer, and it generally pushes support frames toward stiff, short members and closed sections rather than long open beams.

For a buyer, the practical point is this: the fabricator should not change member sizes, add openings or move stiffeners without the designer signing off, because any of those changes moves a natural frequency.

Where the cracks start

Screen support frames do not fail in the middle of a beam. They fail at weld toes, and they fail at the same details every time:

• Stiffener terminations on flanges. A stiffener that ends on a flange creates a notch at the weld toe. Terminating short with a radius moves the detail to a better fatigue category.
• Cope holes and cut-outs. Sharp internal corners in a web are stress concentrators. Radiused copes and ground edges are cheap insurance.
• Bolted connection plates welded to members. The weld around a connection plate is a fatigue detail; the bolt holes near it are another.
• Attachments. Cable tray supports, handrail posts and access platform brackets welded to the frame add fatigue details in places nobody analysed.

EN 1993-1-9 and AS 4100 both provide detail category tables. The drawing should name the category each critical weld is designed to, so that the fabricator knows which welds need full penetration, which need toe grinding and which need MT after welding.

Weld quality for cyclic loading

AWS D1.1 and AS/NZS 1554 both have cyclically loaded structure clauses that tighten acceptance criteria: smaller allowable undercut, no weld profile defects at toes, and specific requirements for weld reinforcement on butt welds in tension. Those clauses only apply if the drawing invokes them. A drawing that just says “weld to AWS D1.1” gets statically loaded acceptance criteria by default.

For screen support frames, buyers usually ask for:

• All primary welds full penetration where the load passes through the weld.
• Toe grinding or TIG dressing on the fatigue-critical fillets, with the extent marked on the drawing.
• 100% MT on fatigue-critical fillets after welding and, where the structure is stress relieved, after relief.
• UT on full penetration butt welds in tension.

Our NDT guide explains what each method finds and what the report should contain. The welding distortion guide covers the sequencing that keeps a stiff frame straight.

Bolted or welded?

Bolted connections between frame members have two advantages for screen supports: they avoid welded fatigue details at the connection and they allow the frame to be shipped in pieces. The cost is fabrication accuracy, because bolted joints in a stiff frame need holes that line up.

The common compromise is welded subassemblies, each small enough to ship and to stress relieve, joined on site with pre-tensioned high-strength bolts. Pre-tensioned bolts in a slip-critical joint do not see the load cycle the way a bearing bolt does, which is why the connection design usually specifies them.

Coating and site conditions

Screen supports live in wet, abrasive conditions. Slurry splash, wash-down water and fines that pack into every corner mean the coating is doing real work. Buyers generally specify a Sa 2½ blast followed by an ISO 12944 C4 or C5 system, or hot-dip galvanising to ISO 1461 for frames that can be dipped.

Galvanising interacts with fatigue in one way worth knowing. The zinc bath is hot enough to relieve some residual stress, but it can also cause distortion in asymmetric weldments and, on very thick or highly restrained welds, liquid metal embrittlement has been reported. The structure’s designer should decide, and the surface finish guide sets out the questions to ask.

What to specify

A fabrication drawing for a screen support structure should state:

• Material grade and certificate level.
• Weld standard with the cyclic loading clauses invoked.
• Fatigue detail categories for the critical welds, and the toe treatment required.
• NDT extent and timing.
• Stress relief, if required, and whether it is before or after machining of any bearing surfaces.
• Bolt grade, pre-tension and hole tolerance for site joints.
• A note that no member size, cut-out or attachment may be changed without designer approval.
• Coating system and the site exposure category.
• Maximum shipping piece size.

Matching the job

Screen support frames are not the heaviest work a fabricator does, but they are among the least forgiving. The shop needs welders qualified for the position and process, procedures that cover cyclically loaded work and an inspector who knows what a toe defect looks like. We check for those before a screen frame drawing goes to a shop. Send yours through the RFQ form and we reply within 24 business hours with which shops fit, and up to three of them quote you directly.