2D Orthographic Drafting of a Plummer Block (Pedestal Bearing) from 3D Views

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1. Functional Overview and Construction of a Plummer Block

A Plummer Block (also known as a Pedestal Bearing) is a heavy-duty split-journal bearing designed to support a rotating horizontal transmission shaft carrying heavy radial loads, particularly when the shaft is not situated close to a machine wall.

                           +-------------------+   Cap / Cover
                           |      Oil Hole     |
                        +--+-------------------+--+
                       /    |  Top Brass (Step) |  \
        Holding Bolt  |     +-------------------+   | Holding Bolt
             |        |    (     SHAFT (d)       )  |     |
             |        |     +-------------------+   |     |
             v        |     |Bottom Brass (Step)|   |     v
           +----+     +-----+-------------------+---+   +----+
           |    |    /                               \  |    |
       ----+----+---+---------------------------------+--+----+---- Sole / Base
       |                                                          |
       +----------------------------------------------------------+
       |<--------------------- Length of Base ------------------->|

Essential Components of the Assembly:

1. Cast Iron Base (Pedestal): Heavy cast frame with two elongated bolt slots (for alignment) in its sole plate.
2. Cast Iron Cap: Retains the upper brass in place; clamped to the base using two square-headed bolts.
3. Brasses / Steps (Split Bearing Bushes): Made of gunmetal, phosphor bronze, or brass to reduce friction and wear. Split horizontally into two halves (Top Brass and Bottom Brass).
   • A rectangular snug on the bottom brass fits into a corresponding recess in the base to prevent the bush from rotating with the shaft.
4. Fasteners: Two square-headed bolts with hexagonal nuts and thin lock-nuts.
5. Lubrication Hole: Centrally tapped oil hole on the top of the cap (for grease cup or wick lubricator).

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2. Standard Empirical Proportions for a Shaft of Diameter $d$

Component Feature	Standard Empirical Formula (IS / BIS)	Typical Dimension for $d = 50\text{ mm}$
Shaft Diameter	$d$	$50\text{ mm}$
Height to Shaft Center	$H_c = 1.05d \text{ to } 1.25d$	$60\text{ mm}$
Total Base Length	$L = 4d \text{ to } 4.5d$	$210\text{ mm}$
Base Width (Depth)	$W = 1.3d \text{ to } 1.5d$	$70\text{ mm}$
Center-to-Center of Bolts	$C = 3d \text{ to } 3.2d$	$150\text{ mm}$
Holding Bolt Diameter	$d_b = 0.25d \text{ to } 0.3d$	$14\text{ mm}$ ($M14$)
Thickness of Base (Sole)	$t = 0.3d \text{ to } 0.4d$	$18\text{ mm}$
Brass Bush Thickness	$t_b = 0.1d + 2\text{ mm}$	$7\text{ mm}$

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3. Systematic 2D CAD Drafting Workflow

When given a 3D isometric projection of an assembled plummer block, follow this structured CAD drafting sequence:

Step 1: Set Reference Grid and Centerlines

• Establish layer 0_Center.
• Draw the main horizontal shaft centerline at height $H_c$ from the base.
• Draw the vertical centerline of symmetry of the plummer block.
• Draw two vertical centerlines for the holding bolts at distance $\pm C/2$ from the center.

Step 2: Draw the Cast Iron Base (Pedestal)

• Draw the bottom baseline of the sole plate (length $L$, thickness $t$).
• Construct the bolt slots ($2 \times \text{Bolt Diameter}$ elongated length).
• Using CIRCLE and TRIM, draw the semi-cylindrical seating cavity for the bottom brass with radius $R = (d/2) + t_b$.
• Add cast fillets ($R = 3\text{ to }6\text{ mm}$) between the vertical pedestal body and sole plate using FILLET.

Step 3: Draw the Split Brasses (Steps)

• Bottom Brass: Sits in the semi-cylindrical base housing. Draw snug ($6\text{ mm} \times 6\text{ mm}$) at the bottom center.
• Top Brass: Identical inner diameter $d$; outer profile has an octagonal or arched exterior matching the cap.
• Draw the shaft bore of diameter $d$ using CIRCLE.

Step 4: Draw the Bearing Cap and Bolts

• Draw the cap contour enclosing the top brass.
• Draw the rectangular stepped bolt-head recesses in the base.
• Draw the square-headed holding bolts (heads captured in bottom recesses to prevent rotation during tightening).
• Add the dual nuts: Standard nut ($T = d_b$) and thin lock nut ($T_1 = 0.6d_b$).
• Insert the central oil hole on the crown of the cap.

Step 5: Sectioning and Hatching (Cross-Sectional Front View)

In sectional assembly drawings (e.g., right-half in section):

• Base (Cast Iron): Hatch with standard $45^\circ$ parallel lines (ANSI31, angle $45^\circ$).
• Cap (Cast Iron): Hatch at $135^\circ$ (opposite direction to distinguish from base).
• Brasses (Bronze/Gunmetal): Different hatching pattern or crossed hatching to denote non-ferrous bronze.
• Shaft and Bolts: Never hatch solid shafts and bolts cut longitudinally!

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4. Key CAD Productivity Commands Used

AutoCAD Command	Functional Application in Plummer Block Drafting
MIRROR	Draft only the left half of the symmetrical base and cap, then mirror across vertical centerline.
FILLET	Apply standard $3\text{ mm}$ to $5\text{ mm}$ casting rounds to all internal and external corners.
HATCH	Fill sectioned metal components with standard ANSI31 hatching at alternating angles.
OFFSET	Create uniform wall thicknesses, brass clearances, and bolt-slot contours.
BLOCK	Insert pre-drawn standard square-headed bolts and hex nuts from a component library.

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5. Summary of Key Exam Points

1. Split Bush Purpose: The bearing is split into two halves so that the shaft can be installed or removed without dismantling adjoining gears or pulleys.
2. Snug Function: Prevents the brass bushes from rotating inside the housing as the shaft turns.
3. No-Section Rule: The shaft, bolts, nuts, and lock-nuts are solid members; they must never be hatched when cut by a longitudinal cutting plane.
4. Hatching Direction: Adjacent parts must be hatched at opposite angles ($45^\circ$ and $135^\circ$).
5. Square Bolt Head: The bolt head is square and fits into an undercut in the base so that it cannot spin when the nut is tightened from above.