Understanding the Envelope Spectrum
The გარსის სპექტრი სიხშირეა? სპექტრი მიღებული გამოთვლით FFT of the envelope — the amplitude-demodulated signal — produced during კონვერტის ანალიზი. It reveals the გამეორების სიხშირე of impacts and modulations buried in high-frequency ვიბრაცია, which makes it the single most powerful technique for detecting rolling-element bearing defects. Where a standard velocity spectrum shows the carrier frequencies — the structural resonances that impacts ring — the envelope spectrum shows the rate at which those impacts occur, mapping directly onto the საკისრების ხარვეზების სიხშირეები BPFO, BPFI, BSF and FTF.
Put simply, the envelope spectrum is to bearing diagnostics what the ordinary spectrum is to დისბალანსი and არასწორი განლაგება: the primary tool that makes early fault detection possible. It pulls clean diagnostic frequencies out of the high-frequency “hash” that a velocity spectrum cannot resolve.
1. How the Envelope Spectrum Is Generated
A localised flaw — a spall on a race, a pit on a roller — strikes a hard contact once per pass and excites the bearing’s natural resonances at several kHz. Those resonances are the გადამზიდავი; the regular train of impacts modulates the carrier’s amplitude. The envelope process strips away the carrier and keeps the modulation:
- ზოლის გამტარობის ფილტრი: isolate a high-frequency band rich in resonance energy (commonly 1–10 kHz), discarding the low-frequency vibration from unbalance and misalignment. A ზოლის გამტარობის ფილტრი does this job.
- Envelope detection (დემოდულაცია): rectify the filtered signal and trace the outline of its amplitude — the envelope.
- Low-pass filter: smooth the envelope to remove any residual carrier ripple.
- FFT: transform the envelope into the frequency domain.
- შედეგი: an envelope spectrum whose peaks sit at the impact repetition rates.
The key idea is that the modulation frequencies recovered by this chain are the bearing fault frequencies. The high-frequency carrier merely acts as a messenger, ringing each time a defect is struck.
2. Reading an Envelope Spectrum
Healthy bearing
- Low overall envelope level.
- A flat or gently sloping trace with no distinct peaks.
- A noise floor at or below the instrument’s sensitivity.
Defective bearing
- პირველადი პიკი: at a bearing fault frequency — ბიპიფო, ბიპიფაი, BSF ან FTF.
- ჰარმონიკები: 2×, 3×, 4× of the fault frequency appear and grow as the defect worsens.
- გვერდითი ზოლები: spaced at cage (FTF) or სირბილის სიჩქარე (1×) intervals around the fault peak, reflecting load modulation as the defect orbits in and out of the load zone.
- Elevated floor: the overall noise floor rises as surface deterioration spreads.
The matching peak tells you which element has failed: a peak at BPFO points to the outer race, BPFI to the inner race, BSF to a rolling element, and FTF to the cage. Because BPFI and BSF rotate through the load zone, they are amplitude-modulated and therefore flanked by sidebands; a BPFO defect in the stationary load zone usually is not.
3. Why It Outperforms the Standard Spectrum
Three properties make the envelope spectrum indispensable for bearing work:
- Early detection: it routinely flags incipient damage many months — often 6 to 18 — before a fault becomes visible in the velocity spectrum, giving maximum lead time for parts and planning. It is sensitive to micro-spalls that produce almost no energy on a velocity scale.
- Clear fault signatures: because unbalance and misalignment are filtered out before demodulation, fault frequencies and their sidebands stand proud against a clean background, far easier to read than a crowded broadband spectrum.
- Low-energy event capture: a tiny impact carries negligible energy at low frequency but excites high-frequency resonances efficiently. Envelope processing amplifies exactly these weak, high-frequency diagnostic signals.
This is why envelope analysis sits alongside the shock pulse method and spike energy as a cornerstone of bearing condition monitoring, and why კურტოზი often rises in step with the envelope level.
4. A Step-by-Step Interpretation Workflow
To turn an envelope plot into a diagnosis:
- Calculate the fault frequencies for the installed bearing — BPFO, BPFI, BSF and FTF — from its geometry and the shaft speed. Our საკისრის დეფექტის სიხშირის კალკულატორი returns all four in seconds, and the ჰარმონიული სიხშირის კალკულატორი helps map the orders.
- Search the spectrum for peaks at those frequencies, allowing roughly ±5% for slip and calculation tolerance.
- Confirm with harmonics — a genuine bearing fault shows a series, not a lone spike.
- Check sideband spacing for additional confirmation of the source.
- Diagnose and grade the defect from the matched element and amplitude.
A rough severity scale, expressed in g of envelope acceleration, helps prioritise action: an incipient defect (≈0.5–1 g) shows a small lone peak — monitor monthly; an early defect (≈1–3 g) shows a clear peak with one or two harmonics — monitor weekly and plan replacement within months; a moderate defect (≈3–10 g) shows a strong peak, multiple harmonics and sidebands — plan replacement within weeks; and an advanced defect (>10 g) shows very high amplitude, many harmonics and an elevated floor — replace urgently. Exact thresholds depend on bearing size and speed, so always interpret them against a machine-specific საბაზისო and your own ტრენდული history.
5. Putting the Envelope Spectrum to Work in the Field
In a მდგომარეობის მონიტორინგი programme, the envelope spectrum belongs on every bearing route: trend the envelope amplitude at each fault frequency and you gain warning far earlier — and far more specifically — than overall vibration trending alone can offer. In troubleshooting it earns its keep when the overall level is high but the standard spectrum is ambiguous, when a bearing problem is suspected, when you must confirm that a replacement is genuinely warranted, or when you need to identify which bearing in a multi-bearing train is failing. A portable two-channel instrument such as the ბალანსეტი-1ა lets a technician capture the high-frequency vibration directly at each housing with an აქსელერომეტრი, so the same field visit that checks ნარჩენი დისბალანსი after a balance job can also screen the bearings for incipient damage.
6. Envelope Spectrum vs Envelope Analysis
The two terms are often used interchangeably, but it is worth keeping the hierarchy straight. კონვერტის ანალიზი is the complete process — band-pass filtering, demodulation and FFT. The envelope signal is the time-domain demodulated waveform, an intermediate product. The გარსის სპექტრი is the final frequency plot, the deliverable an analyst actually interprets. In short, the envelope spectrum is the output of envelope analysis, and it is the gold standard for bearing defect detection: its ability to expose fault frequencies long before they surface in a standard spectrum, paired with clear, element-specific signatures, makes it an indispensable part of any predictive-maintenance toolkit for rotating equipment.
